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Instead of using ArcBluetoothBridge as the pairing delegate and forward
pairing request to the UI dialog, use BluetoothPairingDialog directly,
which will automatically pair and handling incoming requests.

This would also fix the general bluetooth pairing regression which
incorrectly send the pairing request to ArcBluetoothBridge, so default
handling in BluetoothNotificationController is skipped unexpectedly.

Tested two device pairing flow[A] and ble signin flow[B].

A:
1. Get two chromebooks A and B, with B as your DUT.
2. After login on B, start a terminal, type in "bluetoothctl" and then
   "discoverable on".
3. On A, scan for B and then pair with B.
4. Confirm the passcode on A.
5. Confirm the passcode on B.

1. In settings, navigate to manage Android preferences.
2. Click Accounts to add a google account with a ble security key registered
as the second factor authenticator.
3. Follow the steps and observe a UI dialog show up and ask for passcode.
4. Enter the passcode and the pairing is successful (a notification is shown).

B: 
Change-Id: I48ce5d1968f4d532b72af7df5e5b6e7be586d7d8
Reviewed-on: https://chromium-review.googlesource.com/1109377Reviewed-by: Luis Hector Chavez <lhchavez@chromium.org>
Commit-Queue: Xiaoyin Hu <xiaoyinh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#569308}