Add image member to Metadata object

This moves the Metadata object more in line with the available
attributes defined in the AVRCP specification. Image can be
passed to the AVRCP code where they can be updated with an image handle
based on where BIP has stored the iamage. The native stack can use that
handle if one is assigned.

This change does not actually assign any image handle yet. The native
stack also needs to be notified that Java has a BIP client for a given
device before it will actually hand off the handle. This will happen in
future patches.

Tag: #feature
Bug: 153076316
Test: atest BluetoothInstrumentationTests
Test: build, flash, make sure the existing UX is not impacted.

Change-Id: Ic10816f1fd09153418bb57d93a6a2978e4a0f7c3