Bug 977146: Set PROP_BLUETOOTH_ENABLED in ToggleBtAck, r=echou

PROP_BLUETOOTH_ENABLED signals the Bluetooth state to other
components in the system. Setting its value in ToggleBtAck
on the main thread avoids duplicated code in the upcoming
patches.
Bug 977146: Declare ToggleBtAck in header file, r=echou

Declaring ToggleBtAck in the header file of BluetoothService makes
it available ot others classes.
Bug 977146: Push ToggleBtAck into implementations of {Start|Stop}Internal, r=echou

The methods {Start|Stop}Internal for BlueZ and Bluedroid now send
ToggleBtAck to signal completeness of the operation.

The current patch allows for further cleanups in the BlueZ code. It
does not change the semantics or code flow.