Apply FLAG_IMMUTABLE to various PendingIntents.

Some careful spot-checking has revealed common cases where it's
reasonable to blanket apply the new FLAG_IMMUTABLE option to newly
created PendingIntents.  Specifically these situations:

-- Simple notification content clicks are immutable; there's no need
to communicate customized data back to the creator
-- Simple notification action clicks are immutable; there's no need
to communicate customized data back to the creator
-- Broadcast intents sent by AlarmManager are immutable; the system
dispatches them without customization.

Bug: 170165227, 170424283, 170425388, 170425877
Bug: 169791183, 170771965, 170226088, 170224928, 170767530
Test: none
Change-Id: I5ed68710d2ccad4635a30fd91136a9e6ad76a01d