reproducible build for Android apk

[SomberNight]

This attempts to make the Android build process reproducible.

TODOs:

• add some kind of script to compare apks
  - see e.g. https://github.com/signalapp/Signal-Android/blob/dc6dc192dc4c846586152b2b8949aaebf8be5244/reproducible-builds/apkdiff/apkdiff.py

before-merge TODOs:

• buildozer.spec: change "log_level" back to "1"
• submodules: transfer SomberNight/electrum-kivy-atlas to spesmilo
• contrib/android/Readme.md: change fresh_clone instructions to clone from spesmilo

related: #5839

Uses the instructions from kivy/python-for-android#2390 (comment)

Changes for buildozer:

• SomberNight/buildozer@d570116

Changes for python-for-android:

• SomberNight/python-for-android@8856d3e
• SomberNight/python-for-android@1447ea6

[SomberNight]

Status:
I am comparing builds between an Ubuntu 20.04 VM and a rolling Manjaro VM.

• the release-unsigned binaries are byte-identical
• the debug binaries are almost identical; except the apk/zip orders files differently
• I haven't tested signed release binaries yet.

[ecdsa]

great work.
why do you suggest to wait until the 4.2.0 release?

[SomberNight]

We can have it in the next patch release unless we need to do an emergency release before it is ready.
Otherwise, there is no reason to delay it of course.