Use a DeviceConfig property for the default value of the clipboard notification setting.

This adds a DeviceConfig property to hold the default of the new
setting to control whether notifications are shown when an app
accesses clipboard.

Bug: 182349993
Test: manual, "adb shell device_config put clipboard
show_access_notifications false" and observe setting changes when
not set manually; also check notifications shown when expected.

Change-Id: I8c812793db405f0a55aa5ecbc20d893fc4a52bee