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Adds the ability to press the app button via the Chrome VR native
controller input code and switches all uses of app button presses over
to it. This is should fix all the flakiness we've been seeing with
the app button-related tests, as we're no longer relying on the flaky
VrCore-side controller emulation.

As a side-effect, also adds gesture detection capabilities when using
the controller input code since app button short/long presses require
it. This currently only supports button click detection, but could
be easily extended later on to support scrolling, etc.

Bug: 902938
Change-Id: I36dcb3b0b9e34adb92ade8a1a08d0d53536148ce
Reviewed-on: https://chromium-review.googlesource.com/c/1324829Reviewed-by: Christopher Grant <cjgrant@chromium.org>
Commit-Queue: Brian Sheedy <bsheedy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#608112}