Enable clang compilation.

Files under system/bt/device do not use atomic operation.
Old setting of LOCAL_CLANG is unnecessary.

Other atomic operations in system/bt/bt/core were changed
and osi/include/atomic.h was removed, in
https://android-review.googlesource.com/#/c/166762

Change-Id: I807e1fd1792e801392f40ab92a8990c826a5785a