Add a device config for FGS denied notification.

1. Add a flag mFgsStartRestrictionNotificationEnabled to control whether
display a notification when a FGS is restricted by BG-FGS-start. Default
is false.
2. Use device config key "default_fgs_starts_restriction_notification_enabled"
to turn it on/off.

Bug: 187977406
Test: adb shell device_config get activity_manager default_fgs_starts_restriction_notification_enabled
Test: adb shell device_config put activity_manager
default_fgs_starts_restriction_notification_enabled [true|false]

Change-Id: Ie1e1d5351b9f8326a9cb3d6685a8f2cf53978685