Create InputThread to manage inputflinger threads

InputReader and InputDispatcher both manage their own threads. To help
manage input processing threads that have elevated priority/nice values,
we create the InputThread class.

Creating an InputThread starts the thread immediately, and loops the
loop function until the InputThread object is destructed.

Bug: 130819454
Test: atest inputflinger_tests
Test: Touch input works on crosshatch
Change-Id: I2cb56250fc62300e195c5b2474d32c2db3fa4711