android build: bump targetSdkVersion from 30 to 31

[SomberNight]

Re the Android build, this PR bumps the targetSdkVersion from 30 to 31, and upgrades the sdk tools used.
(the google play store will require 31, starting 2022-11 (next month))
A few commits are cherry-picked onto our forks of p4a and buildozer from upstream, but the forks are not rebased.

This was split from the more ambitious #8002, which also tried to rebase the forks.

[SomberNight]

The kivy app builds successfully, but the qml app fails - during compilation of qt5:

Android NDK: Found platform level in /home/user/wspace/electrum/.buildozer_qml/android/platform/build-arm64-v8a/build/bootstrap_builds/qt5/project.properties. Setting APP_PLATFORM to android-31.
Android NDK: android-31 is above the maximum supported version android-30. Choose a supported API level or set APP_PLATFORM to "latest".
/opt/android/android-ndk-r22b/build/core/setup-app-platform.mk:101: *** Android NDK: Aborting.    .  Stop.

It looks like NDK r22b might only support targeting up to sdk 30:

$ cat /opt/android/android-ndk-r22b/meta/platforms.json 
{
  "min": 16,
  "max": 30,
  "aliases": {
    "20": 19,
    "25": 24,
    "J": 16,
    "J-MR1": 17,
    "J-MR2": 18,
    "K": 19,
    "L": 21,
    "L-MR1": 22,
    "M": 23,
    "N": 24,
    "N-MR1": 24,
    "O": 26,
    "O-MR1": 27,
    "P": 28,
    "Q": 29,
    "R": 30
  }
}

I am confused though. I don't understand why the kivy app builds successfully in that case...

---

I will try setting different values for compileSdkVersion and targetSdkVersion:
https://github.com/kivy/python-for-android/blob/11322742d0c9d3d2b7b4642110c911b394425ee1/pythonforandroid/bootstraps/common/build/templates/build.tmpl.gradle#L32

[SomberNight]

I have bumped only the targetSdkVersion in 0efc881, but kept the compileSdkVersion at 30.
I believe this should satisfy the requirements of google play, but this is only a workaround until we can bump the NDK version.