Floss: Fix deadlock between btmanagerd and btadapterd

With Suspend API, btmanagerd calls RegisterCallback to btadapterd and
may be deadlocked waiting for this return. This patch changes the call
to async call so that btmanagerd does not have to block waiting for the
return.

Bug: 232547719
Tag: #floss
Test: Build and run on Chrome OS

Change-Id: I4d245f54dc0684d51c12b7e1698a180eef9b8c58