Enable object manager for btmanagerd

Clients will use object manager's InterfaceAdded/InterfaceRemoved
notifications to keep track of when Bluetooth becomes available.

Also update dbus-crossroads dependency to 0.4, which adds a bugfix for
creating object manager on "/".

Bug: 190606778
Tag: #floss
Test: Verified on ChromeOS
Change-Id: Ib339ea9bf598301efc0f572b35a3ce638039a170