gn-build: Remove all -Wno-unused-* flags from build files

This CL removes the -Wno-unused-variable, -Wno-unused-value, and
-Wno-unused-function flags from all GN build and Android.mk files
(except in test/ & tools/, which are not built using GN yet). Compile
warnings, caused by the newly enabled flags, have been fixed throughout
the code base.

Bug: 21570302

Change-Id: I1c128b62256faa17668124798341005bcd0f7315