Add getBatteryUsageHint() to BluetoothHeadset for power monitoring.

This is a monotonically increasing integer. Wraps to 0 at
Integer.MAX_INT, and at boot.
Current implementation returns the number of AT commands handled since
boot. This is a good indicator for spammy headset/handsfree units that
can keep the device awake by polling for cellular status updates. As a
rule of thumb, each AT command prevents the CPU from sleeping for 500 ms