Incompatible APIs were used in the custom layouts and the extra keys
options.
Add @TargetApi annotations to help catch similar issues in the future
with the help of 'gradle lint'.
Store preferences in device protected storage, which is available before
the device is unlocked.
The keyboard was crashing when trying to access the encrypted
preferences.
The emoji pane uses a separate preferences file, the old data is lost.
The SettingsActivity can't easily use the new preferences storage.
Instead, it continues to use the "default" preferences store, which is
copied back to the protected storage when needed.
