android: misc build fixes

 - Add $(top)/include to the local_c_includes. Required for c99_compat.h
header.
 - Build third_party/threads library, add $(top)/third_party/threads to
local_c_includes (for threads.h)
 - pthread_mutex_timedlock is not available in bionic.
 - Drop unneeded trailing backslash(es).

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Chad Versace <chad.versace@linux.intel.com>