[Development] Dropping support for Android 15 and below in Qt 5.7

[Eskil A. Blomfeldt]

Hi,

According to the data from Google Play, there are currently only 7.4% of 
active devices on Android versions 15 and below (< 4.1).

     https://developer.android.com/about/dashboards/index.html

Since this is a small and diminishing group and since the testing we are 
doing on these old devices is limited, I propose we drop support for 
versions 15 and below in Qt 5.7. It should simplify the code some, and 
definitely our testing requirements.

-- Eskil