Re-enable TLSv1.1/1.2 support

Was disabled in 3fd7470d.

Issue 6238.

Related Android change for API 20:
https://android.googlesource.com/platform/external/conscrypt/+/1f63d2c22326b989105366d2907a83b848dcd29e%5E!/
Update changelog for 4.905
Bumped manifest to 4.905
Add Play Store feature graphic assets
Use android-sdk-manager to fetch Android SDK dependencies
Update HoloColorPicker to use build tools 20.0.0
Add support for disabling pre-dexing
Use checkstyle to monitor code quality
Remove unused strings
Remove unused 'check mark' color chip
Fix code style
Tidy up AndroidManifest.xml

Formatting only; no functional changes
Don't export CoreReceiver
Don't "export" BootReceiver's scheduleIntent action
Remove now unused AccessibleEmailContentActivity
Change/fix indentation of Gradle files
Merge pull request #498 from k9mail/manifest_fixes

Manifest fixes
Re-enable TLSv1.1/1.2 support

Was disabled in 3fd7470d.

Issue 6238.

Related Android change for API 20:
https://android.googlesource.com/platform/external/conscrypt/+/1f63d2c22326b989105366d2907a83b848dcd29e%5E!/
Merging upstream/master, including fixing a merge conflict with my lazy signer changes in build.gradle.