Add a setting to turn clipboard access notifications on/off.

This adds a new switch setting under Privacy to control whether
toast notifications are shown when an app accesses clipboard
(see b/179667029 for the clipboard access notification feature).

Bug: 179667029
Test: make RunSettingsRoboTests ROBOTEST_FILTER=
"com.android.settings.privacy.ShowClipAccessNotificationPreferenceControllerTest"

Change-Id: Ibac7f31538ab3e4d2ec476cb8c05cbbd0c7e6d41