AOSP/DeskClock - Call executePendingTransactions only if mFragmentManager is...

AOSP/DeskClock - Call executePendingTransactions only if mFragmentManager is not destroyed. Fixes the crash bug. Code matches internal code.

BUG: 153590150

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

$ make -j 40

... 6117353 Jul  7 15:27 out/target/product/bonito/product/app/DeskClock/DeskClock.apk

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

Change-Id: I582013ebffef2c8d1ec16255ea531d874d14a968