Add a mutability flag when create a PendingIntent.

It is soon going to be required for apps targeting S+ to specify
explicitly either FLAG_MUTABLE or FLAG_IMMUTABLE when creating a
PendingIntent.
Previously, PendingIntents used to be mutable by default.

Test: atest TelecomUnitTests:MissedCallNotifierImplTest
Bug: 170159387
Change-Id: I4595a5c0ef3e9dcefd531bd45e01db316e5cf318