RFC: NDK r25 LTS updates

[brody4hire]

• drop Jelly Bean support - update minimum API to 19 (KitKat), as needed by NDK beyond outdated r23 ref:
  • https://github.com/android/ndk/wiki/Changelog-r24
• update recommended NDK to r25b ref:
  • https://developer.android.com/ndk/downloads
  • https://github.com/android/ndk/wiki/Changelog-r25

rationale:

• It looks like NDK pre-r25 is no longer supported
• Jelly Bean is now super-old, no longer supported beyond NDK r23, does not look worth supporting too much longer

supersedes other proposals:

• docs: only specify minimum Android version [Jelly Bean] #609 - now included in this PR
• DRAFT RFC docs: update recommended NDK -> r23c #608

✅ tested updated build together with androidx.sqlite/sqlite 2.1.0 (PR #501) in sqlcipher/sqlcipher-android-tests#44 ... looks like several tests need some more compiler flags

I need to build an updated version with some customizations for an organization and may even drop KitKat in my own fork, hoping to keep divergence to a minimum. Happy new year and thanks in advance for your consideration!