InputController: Allow for null InputDevice object from getInputDevice

A null InputDevice object can be returned when there is a race condition
between the device added/changed signal and a removal. There may also be
rare cases where an InputDevice may be hot-plugged upon connection, when
there are race conditions in the kernel APIs.

Bug: 413012407
Flag: EXEMPT bug fix
Test: Presubmit
Change-Id: Ifc21abe6db5e9b712af2f972c759e09d32d80139