DSU enhancement (5/N)

* Support default keyguard strings
  This allows the client to use default strings for the keyguard
  dialog instead of the ones relevant to DSU.

Bug: 277691885
Test: set up pin code and run
  adb shell am start-activity \
  -n com.android.dynsystem/com.android.dynsystem.VerificationActivity \
  -a android.os.image.action.START_INSTALL \
  --el KEY_USERDATA_SIZE 2147483648 \
  --ez KEY_ENABLE_WHEN_COMPLETED true \
  --ez KEY_ONE_SHOT false \
  --ez KEY_KEYGUARD_USE_DEFAULT_STRINGS true
Change-Id: Ic6c528c180a7006d9392734b9a0f562418c47c3b