No production code relies on the mutability. Code in fakeandroid's
AlarmManager added in http://ag/3826144 relies on the mutability,
and backup/internal/PerformInitializeTask sets a (now immutable)
PendingIntent, but fakeandroid uses a different AlarmManager instance
that the real system so these should be unrelated (I think - reviewer,
please double-check my thinking).
Fixes: 170163281
Test: Treehugger
Change-Id: I3dd45cc270c2fd8ab9d4b3402edc6cc5d765fd5a
aa50c2224a