Support associating with an already-paired device

This is required for migration scenario, where device(s) are already
paired(and thus no longer discoverable) but didn't go through companion
flow.
This also fixes a bug with filtering by mac address, which is also relevant to
the use-case of associating a specific device

Test: Pair with a device first, and call associate with a filter with its MAC
address and single device requested. Ensure the device is found.
Ensure only that device is ever returned when filtering by MAC address.
Bug: 62487084
Change-Id: Ic7cc6affc0648ad85b15620e8c3aba4b9fc91aa1