AOSP/DeskClock: Improve AlarmNotification priority to pop up alarm notification.

Root cause: Because alarm notification is IMPORT_DEFAULT in AOSP R,
there is no alarm alert pop up when alarm on time.

Solution: Improve the priority to IMPORT_HIGH.

Merging b/153362039#comment6 and I have added HIGH at one of the missed locations.

Changed IMPORTANCE_DEFAULT to IMPORTANCE_HIGH in showHighPriorityNotification,
updateUpcomingAlarmGroupNotification and updateMissedAlarmGroupNotification methods.

BUG: 153362039

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

$ 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 and waiting for the alarm to go off.

Change-Id: I7d97fe6a39b016a9b9850849b38e1a1b98410807