Add ability to show/hide accessibility_shortcut_preference.

This adds a new boolean flag,
config_show_accessibility_shortcut_preference, which when set to false
will hide the accessibility shortcut preference item.

Bug: 62387367
Test: make RunSettingsRoboTests
ROBOTEST_FILTER=AccessibilitySettingsTest
Change-Id: I3fd941695df1ad89f717f464d2383be909839a3b
5 files changed