Let InputReader handle its own thread

Previously, InputManager created and managed the InputReaderThread,
which interacted with InputReader through the loopOnce() method in
InputReaderInterface.

We move the threading logic from InputManager to InputReader by removing
the loopOnce() method from InputReaderInterface and adding a start() and
stop() method in its place.

Once InputReader is created, InputManager will call
InputReaderInterface::start(), which creates and starts InputReader's
own thread. InputManager can also call InputReaderInterface::stop() to
halt further processing on InputReader's thread.

Bug: 130819454
Test: atest inputflinger_tests
Test: Touch input works on crosshatch
Change-Id: Ic732436d4f00a831e317be1f16ac106a11652cff