* Android: Segmentation fault fix, PendingIntent flag, and other fixes
- Information about the crosshair is sent after camera initialization.
- Since API 31, PendingIntent requires mutability flag.
- super (class) is called in onRequestPermissionsResult().
- GameActivity suppresses "unused" warning since most of its methods are called from native code.
- Non-null safety is added for nullable function calls.
- Warning/error logging is added for various function calls' return value.
* Move utility functions into Utils.java
- Some nullable functions are changed to be non-null functions.
- Some null checking outside it is removed.
- More annotations are added to functions and parameters.
From November 2021, the Play Store will no longer be accepting
apps which use the deprecated getExternalStorageDirectory() API.
Therefore, this commit replaces uses of deprecated API with the new
scoped API (`getExternalFilesDir()` and `getExternalCacheDir()`).
It also provides a temporary migration to move user data from the
shared external directory to new storage.
Fixes #2097, #11417 and #11118
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