Android update

• Updated to the latest OpenConnect v9.0.1+
• Now works with Android 12 and 13, ready for 14
• Android NDK updated to r25c
• Gradle updated to 8.0 (works with the latest Android Studio 2022.02 Flamingo)
• Minimal SDK is now 23 (Android 6.0 Marshmallow, 98.2% of cumulative usage)
• 8.0+ devices now can autofill username/password.
• Java errors fixes
• make -C external now should work

Resolves: #3 #4 #5

Related MRs: openconnect!464 build-images!30 (merged)