Reland "Let InputReader handle its own thread"

This CL was first landed in Ic732436d4f00a831e317be1f16ac106a11652cff
but was reverted due to flaky tests.

The flaky tests were caused by races between the instrumented test
classes and the InputReader class under test, which now runs in a new
thread.

In addition to re-landing the change, this CL ensures that InputReader
does not start its own thread in its unit tests, but instead keep using
the loopOnce method that is exposed for testing.

Bug: 130819454
Test: atest inputflinger_tests
Test: Touch input works on crosshatch
Change-Id: I5bbfc68b6be6745efc8b7106a9292ee3cb431c9c