DeskClock: Add FLAG_IMMUTABLE flag to PendingIntent

Fixes:
Targeting S+ (version 31 and above) requires that one of FLAG_IMMUTABLE
or FLAG_MUTABLE be specified when creating a PendingIntent.

Strongly consider using FLAG_IMMUTABLE, only use FLAG_MUTABLE if some
functionality depends on the PendingIntent being mutable,
e.g. if it needs to be used with inline replies or bubbles.

Change-Id: I63b25512d3bf6e0d3b0cc95d91295f4065175848