accelerator_controller: Use new way to change volume

Use IncreaseOutputVolumeByOneStep in HandleVolumeUp event instead of increase 4% volume everytime.

Use DecreaseOutputVolumeByOneStep in HandleVolumeDown event instead of decrease 4% volume everytime.

Add feature flag to protect volume granularity feature

BUG=b:215297837
TEST=./out/Default/ash_unittests --gtest_filter="*AcceleratorControllerTest*"
TEST=./out/Default/unit_tests --gtest_filter=AboutFlagsHistogramTest.CheckHistograms
TEST=./out/Default/unit_tests --gtest_filter=AboutFlagsTest.EveryFlagHasMetadata

Change-Id: I82a63b52008d914e46699e7a073388b468bc3b43
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3802344
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Commit-Queue: Ching Yun Chang <whalechang@google.com>
Reviewed-by: Hsinyu Chao <hychao@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1039538}