graphics: fix use-after-free in mapper 2.0 passthrough

The mapper 2.0 gralloc passthrough keeps a set of the native_handle_t
pointers of all imported buffers. This change ensures that the
underlying HAL wrapper freeing the native_handle_t is performed
atomically with respect to accesses to the passthrough's native_handle_t
pointer set.

This fixes a race where a native_handle_t could be reallocated between
being freed by the HAL wrapper and removed from the passthrough's set,
which could then cause the import which happened to reallocate that
native_handle_t to spuriously fail.

Bug: 170798776
Test: Manually very no more spurious import failures in ARCVM's decoder.
Change-Id: I654a44e94adb319f54fb462f4484db414ca1b154