SystemMediaRoute2Provider: Set callback after instantiation is done

A NullPointerException can happen if some callback method is called
before the constructor of SystemMediaRoute2Provider ends.
This CL fixes the issue.

Bug: 158838070
Test: Passes CTS and mediaroutertest
Change-Id: Ibad3c54ad3f7921621628b6147c35605652da29b
2 files changed