DeskClock: Fix alarm clock screen show black when screen locked;

Root cause: API ActivityTaskManagerService.notifyKeyguardFlagsChanged is called multiple times,
which causes the activity status abnormal.

Solution: Add 'android:showWhenLocked = true' to
ensure activity show on the top even  when screen locked.

BUG: 156311117

Test: manual - Tested the DeskClock UI manually and tested the alarm.

$ make -j 40
$ ls -l  out/target/product/bonito/product/app/DeskClock/DeskClock.apk
  -rw-r----- 1 rtenneti primarygroup 6117353 Jul 13 11:35 out/target/product/bonito/product/app/DeskClock/DeskClock.apk

$ adb install -r out/target/product/bonito/product/app/DeskClock/DeskClock.apk

+ Verified by setting up the alaram, lock the screen and waiting for the alarm to go off.

Change-Id: I69c44571cd86c21203311c89081c8935d5ed1450