bluetooth: Check if Object Manager is supported before initializing clients on linux

Two big changes:

1. On Linux, we check if Object Manager is supported before we try to initialize
   the DBus clients. This solves the log spamming issue. On Chrome OS we initialize
   the clients regardless of Object Manager support (we've always done this);
   we keep the current behavior because by the time BluezDBusManager is
   initialized Bluez is not ready and it appears as if Bluetooth is not supported

2. BluetoothAdapterBluez::CreateAdapter now takes a callback which runs after
   we know whether Object Manager is supported or not and BluetoothAdapter is
   initialized. Because of this change we fix all tests and users that assumed
   BluetoothAdapterFactory::CreateAdapter was synchronous.

Design doc: https://docs.google.com/a/chromium.org/document/d/1xmMV7RsnEM6bejH-fYejAdE7q4BkucutuVFP7WI7mgY/edit?usp=sharing
BUG=583637

Review URL: https://codereview.chromium.org/1679983002

Cr-Commit-Position: refs/heads/master@{#377772}

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