Make BluetoothHeadsetClient and associated functions @SystemApi

New System APIs include:
- getConnectionState
- getConnectedDevices
- getDevicesMatchingConnectionState
- getConnectionPolicy
- setConnectionPolicy
- ACTION_CONNECTION_STATE_CHANGED

Bug: 206035301
Test: atest BluetoothInstrumentationTests; atest BluetoothHostTest;
Change-Id: Ica41ab7e19ec66e866e571c7e2c0fc0b6b67a987