Hide warning in cutils/trace.h

system/core/include is included in the global include path using
-isystem, which hides all warnings.  cutils/trace.h has an ignored
return value warning, move the implementation to trace-dev.c so it
doesn't cause warnings in every module that includes it in preparation
for moving from -isystem to -I.

Test: m -j native
Bug: 31492149
Change-Id: If8b3fe13059c9e59c2d5208294d427d84fa6e588