[ScreenLock] Fix result code handling

Modify the back stack and result code propagation in the screen lock
scenarios.

- EncryptionInterstitial now propagates the result of ChooseLock*
  request instead of always returning RESULT_OK.
- ChooseLockGeneric now treats CHOOSE_LOCK_REQUEST and
  ENABLE_ENCRYPTION_REQUEST the same (since encryption can be a proxy
  for ChooseLock*). This means ChooseLockGeneric will now stay on
  back stack when going back from ChooseLock*, just like the case
  (indirectly) through EncryptionInterstitial.

Bug: 26177240
Change-Id: Id7f1256dcbff00d552a3e7db60c285f53f1e63e6