[Fast Pair] Check devices are connected on DevicePairedChanged events

Fast Pair is creating MessageStream connections to disconnected
devices as a result of DevicePairedChanged geting called when the
adapter first becomes available. When the first connection request
to a device is called for the MessageStream UUID, it prevents other
profiles from automatically connecting.

To prevent Fast Pair from creating the first connection attempt,
a check has been added to make sure DevicePairedChanged events only
attempt MessageStream connections for connected Bluetooth devices.

Bug: b/303119640
Test: Updated unittests
Change-Id: I593705dd9babb2254c3b486fa6625d8aa1d3876d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5400199


Commit-Queue: Jack Shira <jackshira@google.com>
Reviewed-by: Brando Socarras <brandosocarras@google.com>
Cr-Commit-Position: refs/heads/main@{#1279233d}