Use DeviceConfig property for default value of clipboard access notification setting.

This uses a new DeviceConfig property for the default value of the
setting that controls whether a notification is 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.
Test: make RunSettingsRoboTests ROBOTEST_FILTER=
"com.android.settings.privacy.ShowClipAccessNotificationPreferenceControllerTest"

Change-Id: I452b1850ecc5fefb2ca9476d249866b32c885478