Let InputReaderContext hold a single PointerController

InputMappers obtain the pointer controller directly from the policy,
which, when we support multiple pointer controller, makes it natural to
have one pointer controller for an input device.

Rather than that, we want to build a system that makes it more natural
to have one pointer per display. To give more control over the
management of pointers, we make it so that InputReaderContext is
responsible for managing the pointer controller.

Bug: 146385350
Test: atest inputflinger_tests
Change-Id: I0b9276ed4a7f55f04f910dd484ecc7f767b8723b