Lookup layer handle when registering region sampling listener

We must do this in order to prevent clients from providing a bogus
handle when registering a region sampling listener. Fortunately, this
particular path required a permissions check so it cannot be accessed
from arbitrary apps on unrooted devices. But, we should not allow this
type of memory corruption to be reachable by the system.

Bug: 153467444
Test: libgui_test
Test: Repro steps in the bug no longer reproduce
Change-Id: I883506798574dfd0688371fdb6305cfad9d153fc