floss: Refactor btstack initialization order (1/4 SocketManager)

This patch does:
- Move SocketManager creation later than Bluetooth init, so we can
  obtain the socket profile interface right there. This helps avoid
  `Option` in SocketManager.
- Move the btstack construct/init code closer - They don't need to be
  separated in 2 places.

Some backgrounds:
  We want to get rid of the redundant `Option` of the topshim objects
  that is causing many verbosities and confusing snippets (We all know
  we can't do anything without the objects... Then why is it optional?).
  The overall direction is to remove all Bluetooth's dependencies on
  initialization, because the Bluetooth topshim object needs to be
  initialized first before all other topshim objects can be obtained.

Bug: 254870880
Tag: #floss
Test: mmm packages/modules/Bluetooth
Test: manual NearbyShare / PhoneHub / Allegro WCA
Flag: EXEMPT, Floss-only changes
Change-Id: I3293d0970dd20e751da928fab09b560e448410c8