Make OnBackInvoked callback a WeakReference.

OnBackInvokedCallbackWrapper instances are sent to the WindowState over
AIDL. This leads to Activity leak issues even if the reference is
cleared in the remote side. This change makes the held reference weak
So it is not retained.

Test: verified leak no longer present through hprof
Bug: 221285764
Change-Id: I1c529d27769e2426898776e47f37bdb7abe1f681