PendingIntentTest: Add tests to check enforced mutability flag
requirement

Apps targeting S+ are now required to specify an explicit mutability
flag (FLAG_MUTABLE or FLAG_IMMUTABLE) when creating PendingIntents. THis
change adds a few tests to verify that an IllegalArgumentException is
thrown if a mutability flag is not specified when creating a PI.

Bug: 160794467
Test: atest PendingIntentTest
Change-Id: I8598ab0f3a45e68089de95d6a6b04e8952a74321