Bluetooth: mark all Paired devices as Trusted

Move the point in which we mark a device as Trusted (incoming
connections allowed) from the completion of the Pair() command to
the Paired property becoming true.

This means that devices paired from an incoming pairing request
will now also be trusted.

BUG=349608
TEST=device_unittests

Review URL: https://codereview.chromium.org/194713014

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@256499 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 256499