Let InputFlinger create the server InputChannel

We can be sure that server channel won't leave InputFlinger.

Bug: 167947395
Test: Touch events are still dispatched.
Test: m -j inputflinger
Test: atest inputflinger_tests
Test: atest inputflinger_benchmarks
Test: atest libgui_tests
Change-Id: I35f768c8a7b3bfe11e04577c028e52556b60ff14