update ndk version to r20 - preparation for go libraries cross compiling

See merge request !92

update help instructions to prepareForDistribution.sh and add option to build current HEAD instead of an officially tagged release

update build script for building and signing stable and beta releases of Bitmask and custom branded apps

Improve blocking vpn

See merge request !91

>> avoid race conditions between sharedPreference writing and onBoot receiver reading
* don't explicitly exclude Bitmask from routes

Ui tweaks

See merge request !90