log.h: wrap android_testLog around __android_log_is_loggable

- take android_testLog from the bonepile and resurrect
- android_testLog is used, and makes sense in its current context
  as a holder for Android Logging policy.
- Default level when not specified is ANDROID_LOG_VERBOSE for
  eng and userdebug, and ANDROID_LOG_DEBUG for user.

NB: Properties, which are associated with __android_log_is_loggable are
not to be adjusted in user release; it would be useful at that juncture to
return a constant of (0) for VERBOSE, and a (1) for others on user builds.
Or default level to drop to ANDROID_LOG_DEBUG for all in future.

Bug: 17760225
Change-Id: I420b5b8a94d53d664c6a8b13546ebc424a147923