Floss: Clean up warnings

Fixed several warnings so that we can see real warnings at development:
* Removed unused `dbus_connection` in `ManagerContext`
* Change topshim functions to be public
* Prefix topshim internal fields with _ until it's read in future code
* `get_connectable` and `set_connectable` utils should be public

Bug: 189497489
Tag: #floss
Test: Build floss on Linux and check for warnings

Change-Id: I79cd981000d54a502c2d5c2511844e91c9ca58bb