Mark functions in headers with static inline

Functions in headers that are not static inline will cause duplicate
symbol errors when linking if the header is ever included twice
into different compilation units of the same module.  It also causes
-Wunused-function warnings if they are not used in the cpp file, which
were hidden by the use of -isystem frameworks/av/include.

Bug: 31751828
Test: m -j checkbuild
Change-Id: I36f258bb4f5c3df54e874280beab344e63c519d5
(cherry picked from commit 413adafb)