Android expert is needed for getting source code from APK and do some modifications within the app (vibration, dark screen, button function mod.)

I have an APK that was made with Android Studio (see it attached). Basically it is a timer, that saves the user's click time intervals with some other infos into a DataBase. The basic functions are working, see the 'Screen design [login to view URL]' for the original specifications. The following should be improved: - 1 vibration when entering into 'DIRECTION' (Main Screen). 2 vibrations when entering into TIMER Screen. 3 vibrations when entering into AUTO Saving screen. - Vibration should be turned on/off in Options screen (the button is already there, the function has to be added). - Vibration function might be needed when user is pressing buttons on Main Screen, Timer Screen, Auto Saving Screen too. - DARKEN function (see attached image 'Main Screen with DARKEN function [login to view URL]') should work like the following: ALL the functions in Main Screen, Timer Screen, Auto Saving Screen are working (buttons, time measurement, Vibrations, etc.), BUT with totally Black Screen. It should be taken back on Main Screen by pressing 'OFF' (the only button that is visible in this state). - AUTO Saving (Add Value Screen / 3rd Screen): buttons should work like in the other 2 (Main and Timer) screens, the timing should be removed (it is now working like: user can push (icons on the) button only for a limited time, Left and Right, alternately). The right solution should be: user can push the whole button, any button, any time. BUT: if no button is pushed clicked for example 4s (defined in Options Menu, Auto Saving delay), the Data is saved into DataBase. - AUTO Saving screen: the maximum of '1st VALUE' (left number), and '2nd VALUE' (right number) should be limited to 9. If user clicks more, the counter should start again from 0. Like this: 0-1-2-3-4-5-6-7-8-9-0-1-2-3-... etc. - Please give me the availability of the DataBase too (if it can be defined from the APK). Exactly where is it stored on the phone? - Finally: I need both the Source Code and APK version of the improved application.