Add accessibility usage preference to the Privacy screen.

Bug: 123595964
Test: Open Privacy page with one and two accessibility services
enabled and click the preference.
Test: Open Privacy page with zero accessibility services enabled and
do not see the preference.
Test: atest AccessibilityUsagePreferenceControllerTest

Change-Id: Ib1dab374cb53dfbc473b2d5ac49115e608a13ab8
4 files changed