Specify FLAG_IMMUTABLE when creating PendingIntent

Targeting S+ (version 31 and above) requires that one of
FLAG_IMMUTABLE or FLAG_MUTABLE be specified when creating a
PendingIntent. So this CL uses FLAG_IMMUTABLE to create
the PendingIntent because the flag is recommended in most cases.

Bug: 192103710
Test: manual
Change-Id: I5a03b7839fd85ea347f89f076c0f80398e784ade
2 files changed