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This change enables all warnings and treats warnings as errors. As
a result, nobody should be able to submit new code to bluedroid
that contains compiler warnings. Instead, they should fix the warnings
before submitting.

This change has already caught a type mismatch bug in
stack/avrc/avrc_api.c where an integer was being stored in a boolean.

Change-Id: Ic41960754b39a1a6aa37f1c1095003195f466183