The PIC transmit the frequency in 5 bytes to the USB-UART adaptor, the Android device is connected to the USB-UART via OTG cable or adaptor.
The PIC16F628A generates an accurate 1 second time base for the counter by timer1 and CCP. The counter measure the frequency for 0.1sec first, if the frequency is greater than 2MHz TMR0 prescaler divides by 4 to increase the input frequency range to 20MHz with resolution of 4Hz, if the frequency below 2MHz TMR0 is set for no prescaling and the measurment is with resolution of 1Hz.
The app is in free program Basic for Android (B4A) https://www.b4x.com/.
OTG (On The Go) is the cable that is used to connect USB devices (other than charger or PC) to phones or tablets.
The app is an APK file, transfer to the Android device and opening the file gives an option to install it. The code is provided if you wish to modify the app, it is in a B4A free program for PC.
USB-UART can be bought from eBay: https://www.ebay.co.uk/sch/i.html?_from=R40&_trksid=p3519243.m570.l1311&_nkw=usb+to+uart

Frequency Counter:
The Arduino generates an accurate 1 second time base for the counter by cascading timer0 and timer2. The link between digital inputs 3 and 4 connects the output of timer2, 250 Hz, to input of timer0. The software waits Frequency counter for Android using Arduino Nano or Uno.for the output of timer0 to go positive to start the count of frequency input to timer1. Timer1 is a 16 bits timer, it overflows at the count of 2 power of 16, that in turn advances over-flow register. At the end of the 1 second the 16 bit register is recorded. The Arduino sends via OTG cable or adaptor 3 bytes that contain the measured frequency. The specs recommend input up to 6 MHz, I tested it at 2 MHz.
Voltmeter:
It's good to talk. Voltmeter app in B4A using Text To Speech. Arduino connected to Android via OTG cable.
Voltage input is at pin A0, max 5V. The talking voltmeter has an adjustable timer to set the talk intervals (5 to 105 sec) and a stop button.
Optional pushbutton at pin 2 of the Arduino triggers the talk.

OTG (On The Go) is the cable that is used to connect USB devices (other than charger or PC) to phones or tablets.
Speach is in the default language of the device.
The app is an APK file, transfer to the Android device and opening the file gives an option to install it. The code is provided if you wish to modify the app, it is in a B4A free program for PC.

The Arduino generates an accurate 1 second time base for the counter by cascading timer0 and timer2. The link between digital inputs 3 and 4 connects the output of timer2, 250 Hz, to input of timer0. The software waits Frequency counter for Android using Arduino Nano or Uno.for the output of timer0 to go positive to start the count of frequency input to timer1. Timer1 is a 16 bits timer, it overflows at the count of 2 power of 16, that in turn advances over-flow register. At the end of the 1 second the 16 bit register is recorded. The Arduino sends via USB cable and OTG cable or adaptor 3 bytes that contain the measured frequency. The specs recommend input up to 6 MHz, I tested it at 2 MHz.The Android app display 0 to 9 images of nixie tubes.
OTG (On The Go) is the cable that is used to connect USB devices (other than charger or PC) to phones or tablets.
The app is an APK file, transfer to the Android device and opening the file gives an option to install it. The library is needed if you wish to modify the app, it is in a B4A free program for PC.

The Android app is made using MIT App Inventor which is an easy way to make Android apps. http://appinventor.mit.edu/explore/ . The app has
buttons to connect and disconnect the Bluetooth to the phone and 2 displays of voltage and frequency. The .apk file is the file that installs
manually the app to your phone. The .aia file can be imported after you install the MIT App Inventor.
ARDUINO
The Arduino generates an accurate 1 second time base for the counter by cascading timer0 and timer2. The link between digital inputs 3 and 4 connects the output of timer2, 250 Hz, to input of timer0. The software waits for the output of timer0 to go positive to start the count of frequency input to timer1. Timer1 is a 16 bits timer, it overflows at the count of 32768, that in turn advances overFlow register. At the end of the 1 second the 16 bit register is recorded. The Arduino then sends to the Bluetooth 5 bytes of data in serial UART protocol.
HC-05 is pre-set to code 1234 and name HC-05. The module UART is pre-set to 9600,n,8,1.
PIC16F690
This project is for the Android app and HC-05 as above. The PIC counts the frequency over 1 second period generated by CCP1 and Timer1. Input is counted by Timer0 that can count up to 4 MHz. After each count period 2 bytes of ADC and 3 bytes of the counter are transmitted to the HC-05 Bluetooth module.

You are free to use the circuit diagram and the software with no limitations.