Floss: Request Manager DBus name when ready

Some clients immediately call btmanager APIs after the service can be
seen, which can fail if the object manager or interface isn't fully
initialized. While we could change client behavior, modifying
btmanager seems the least disruptive solution.

Bug: 359815670
Test: mmm packages/modules/Bluetooth
Test:stop ui; stop btmanagerd; start ui; start btmanagerd; make sure
Bluetooth Icon exists.
Flag: EXEMPT, Floss-only changes

Change-Id: Iebf1e6d079ad17160105ecb2b38e80b432df2731