Preventing NPE crash when binder call isOnBackInvokedCallbackEnabled

The IME on back dispatcher may call the host context after the
context is deleted, checking whether the host context exists before
accessing it.

Flag: EXEMPT bugfix
Bug: 381495419
Test: presubmit
Change-Id: I9dc1b2d26b8f6b37a7eb57222bb9141f6d145f27