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
Remove option to select (legacy) APG as crypto provider
Remove legacy APG interface
Remove APG permission
Remove unused strings
Merge pull request #498 from k9mail/manifest_fixes

Manifest fixes
use interfaces, not implementions
Use collections instead of arrays to enable stronger typing and reduce cruft
use isEmpty instead of size() == 0 for clarify
removed unneeded variables and clarified names
simplify following code review
EnumSet is more efficient than HashSet
Revert accidental change
Set interface makes more sense for flags
Revert "Implement SSL file-based session caching"

This reverts commit 43c38a047feedda4720af5bfbc188a33f8dfaced.
Change text in feature graphic