Adapter: re-order init to prevent null exception

As soon as the callback are registered, the adapter can be notified.
There is a thin possibility that the bluetooth is being started at the
same time which would result in the callback being called before the
initialization is completed.

Since the initialization is only creating local variable except for the
registration, moving the registration to be the last init action would
solve this.

Note that this require a very fast callback call and a very slow init
altogether with a precise timing.
None of that are expect on a normal journey

Bug: 376735216
Fix: 376735216
Test: m .
Flag: Exempt, trivial fix
Change-Id: I6029fb30a7c4bf3be6d10b743684b6bbc41b5cb1