gralloc: Allow any process to have multiple buffer handles

Change-Id: I670ff7e03b19c07094b506a4422f99c30f06db0e
gralloc: Allow unmapping concurrent handles

If we allow registering those buffers, we need to let them go, too

Change-Id: I6c2dc862e1e783e2b0d3c7d37d60ebe2290173fa