Use -Werror in frameworks/av/media/libaaudio

* Remove unused local variables, labels, functions, private fields.
* Fix warning of unused return result.
* Use template functions in .h file to avoid warning of unused static functions.

Bug: 66996870
Test: build with WITH_TIDY=1
Exempt-From-Owner-Approval: to fix broken build
Merged-In: I54962a7fd36a405e54366130b9c0a24e26fe8ebb
(cherry picked from commit 68326fe0)

Change-Id: If5804a74ca2c0560debd56407d8d1c1a25e11129
(cherry picked from commit 9bba7f17)