Floss: Set up handling of D-Bus methods early in btmanagerd

Setting up the D-Bus method handler needs to be done before exporting
interfaces, so that when the InterfacesAdded signals are sent, we are
ready to handle D-Bus method calls immediately.

This uses the same principle as the fix in btadapterd:
https://r.android.com/2105568.

Bug: 263432564
Tag: #floss
Test: Build Floss on Linux

Change-Id: Ia99e7400a47cde5fc5e0f037ea4c6950b4a7e875