Make BluetoothHeadsetClient and associated functions @SystemApi

The BluetoothHeadsetClient class is currently @hide. This CL annotates
it to @SystemApi. This CL also adds explicit @hide to all public class
members that we're not ready to commit to at this time.

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

Tag: #feature
Bug: 206035301
Test: atest BluetoothInstrumentationTests; atest BluetoothHostTest;
Change-Id: Ib47efd90ad75309cab04e16720d2b172e7cceab1