Never restricts Accessibility Activities based on ECM/admin.

Keeps accessibility activity preference as inheriting from
RestrictedPreference since it shares lots of behavior with the
accessibility service perference (same base class), but always
calls setEnabled(true) for activities.

Fix: 331990900
Flag: com.android.settings.accessibility.never_restrict_accessibility_activity
Test: atest RestrictedPreferenceHelperTest
Change-Id: I39971bc3f65aa630fa62a0e31132f177fd21b635