GitLab

By exposing several apis in input_device_settings.h and add new local pref for TouchScreenEnabled, we can disable touch screen events when backlights are forced off so that no screen UI events can be triggered and enable touch screen when stopping backlights forced off is called properly.

BUG=655388
BUG=633304
TEST=manual test on device, and add test coverage.
also test that this added behavior is compatible with toggle touchscreen shortcut
The shortcut induced in https://codereview.chromium.org/2467023004 is per user pref, backlights-forced-off related touch screen status is per device pref. The final hardware touchscreen status is determined by the rule: enabled_in_local_state && enabled_in_user_pref

Review-Url: https://codereview.chromium.org/2533373002
Cr-Commit-Position: refs/heads/master@{#436177}