BluetoothAdapter: add ability to set default pairing delegate

The default pairing delegate will be used for incoming pairing requests.
Permit multiple to be registered so that the UI can have one for popup
notifications, and a JS application could register one at a higher
priority, so the JS application will be favored.

BUG=338492
TEST=device_unittests included

Review URL: https://codereview.chromium.org/170063008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@253715 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 253715