Initialize AccelerometerReader using a SequencedTaskRunner
Change the initialization of the AccelerometerReader to occur using a SequencedTaskRunner. Updated the subsequent reads of the accelerometer file to also use the same SequencedTaskRunner.
This way reading and processing accelerometer values doesn't affect UI.
It already notifies observers on their own threads. So items which need to
update UI receive the updates on the UI thread.
TEST=manual testing on device
BUG=461433
Review URL: https://codereview.chromium.org/
1174193002
Cr-Commit-Position: refs/heads/master@{#335711}