Removed unnecessary calls and creations of Binder

The previous behavior registered HotspotController to the WifiManager
every time a callback was added in order to trigger an update in the
number of current connections. This caused two things:
* Every registering with WifiManager creates a new Binder object (see
WifiManager#registerSoftApCallback)
* Every callback added triggered Callback#onHotspotChanged on all the
current callbacks

This removes both behaviors while maintaining the information sent to
the callback on addCallback by posting an update. We assume that the
information currently in HotspotController is up to date, if not, there
would be a callback.

Test: atest
Test: manual (opening and closing QS)
Fixes: 123546508
Change-Id: Ica73082beb752a7c20e11b2b2a441fa84bf948a1