Upgrade buildToolsVersion & targetSdkVersion & compileSdkVersion to Android 12

[emanuelb]

Currently values used are for Android 10 & old buildToolsVersion:

bither-android/build.gradle

Lines 28 to 29 in c4f4a68

	compileSdkVersion 29
	buildToolsVersion '28.0.2'

bither-android/build.gradle

Line 33 in c4f4a68

targetSdkVersion 29

bither-android/bither-android/AndroidManifest.xml

Line 11 in e4d9aa8

android:targetSdkVersion="29"

The values for Android 12 are:

compileSdkVersion 31
buildToolsVersion '31.0.2'
targetSdkVersion 31
android:targetSdkVersion="31"

Behavior changes for Android 12:
https://developer.android.google.cn/about/versions/12/behavior-changes-all
https://developer.android.google.cn/about/versions/12/behavior-changes-12