Expose Vendor Specific Headset Event Intent.

The Bluetooth Headset spec doesn't cover all cases.
For example, there is nothing defined to show the battery status of
the headset on the phone. This intent allows 3rd party applications
to communicate effectively with vendor headsets. For example, Plantronics
has an app for Android headsets and when used with Plantronics
headsets, they will be able to use extra information from the headset.

Change-Id: Ib997327103cbbe57d64c52ba27f74162b6769798