Update InputListener usages in InputManagerService

The input components such as InputReader, InputDispatcher are no longer
returned as pointers.
We never checked them for nullness previously, so the code using
references will be more correct.

Bug: 198472780
Test: adb shell monkey 1000
Change-Id: Ie1239cd42ac6bfa9219e4b0cc87b205a412da0e8