PointerController: Add guards to ensure display is valid

This change makes it so that PointerController does not ask its Policy
to load any resources for any displays until a DisplayViewport is set,
and verifies this with unit tests.

Bug: 145699789
Bug: 146385350
Test: atest libinputservice_test
Change-Id: I2e48e7ac4700e6f9fdf939a7bd0e6639b051ade6