Notify user when waiting for decryption password

1. Disable back presses from physical keyboard during encryption: Fix b/6139810
2. Keep screen on when waiting for password. Fix b/6153213 and b/6149606
3. Alert the user with sound when waiting for password. Fix b/6149606
4. Add debugging feature to display the password screen without having to reboot the device.

Change-Id: I588aa7d96e1140f95a6fa91e0281117907f666f7