Add strings for battery level info of connected devices

* SystemUI: quick_settings_connected_battery_level
* SettingsLib:
	bluetooth_connected_battery_level
	bluetooth_connected_no_headset_battery_level
	bluetooth_connected_no_a2dp_battery_level
	bluetooth_connected_no_headset_no_a2dp_battery_level

Bug: 63027849
Test: make, test using these strings
Change-Id: I9052dad87d2afd30d005d050c6b3bfc4813853df