mcp: Fix possible null pointer exception

There is a tiny time gap between the MediaControlProfile construction
and the asynchronous init() method call, in which the ListCallback
methods can be called. In such case and exception is thrown.
This change moves the callback registration from the constructor
to the init() routine, right after the member variable initialisation,
to guarantee it being not-null on the first callback call.

Fixes the following exception:
  java.lang.NullPointerException: Attempt to read from field
  'com.android.bluetooth.audio_util.Metadata
  com.android.bluetooth.audio_util.MediaData.metadata' on a
  null object reference at
  com.android.bluetooth.mcp.MediaControlProfile$ListCallback.run(MediaControlProfile.java:82)

Test: atest MediaControlProfileTest
Sponsor: jpawlowski@
Bug: 150670922
Bug: 199499736
Change-Id: Ie8946701b49d6540bbba3d8edad6eb24269263cc