Flagging: Wrap static flag usage

Provide macro to use static flag while not having to deal with floss not
supporting it.
Usage is like follow:
      #include <android_bluetooth_flags.h>
      if (IS_ENABLED(myStaticFlag)) {
        // new code
      }

Bug: 309697497
Fix: 309697497
Test: m Bluetooth | build change does not impact device code
Change-Id: I660b8e772b36b261da534d918a2d6f46e8ed3863