Refactor to support default Bluetooth pairing delegate

In order to support a default pairing delegate we need to move the agent
service provider delegate implementation from BluetoothDevice to
BluetoothAdapter while retaining the existing API.

BUG=338492
TEST=device_unittests, unit_tests, browser_tests

Review URL: https://codereview.chromium.org/148293003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@252216 0039d316-1c4b-4281-b951-d872f2087c98
void BluetoothAdapterChromeOS::RemoveObserver(
    BluetoothAdapter::Observer* observer) {
  DCHECK(observer);
  observers_.RemoveObserver(observer);
}

void BluetoothAdapterChromeOS::RemoveObserver(
    BluetoothAdapter::Observer* observer) {
  DCHECK(observer);
  observers_.RemoveObserver(observer);
}
