liblog: don't use a cache for __android_log_is_debuggable()

ro.debuggable is set by init before other processes would possibly
call __android_log_is_debuggable(), so this property can be checked
once and stored without liblog's property caching mechanisms.

Test: this function returns accurately
Change-Id: Idb1e5e10e53ca809b7f9a75164a4982f39827d4e