Fix finding media button session

Problem was cuased by previous CL
9fa9b10a [Media ML] Add get/setSessionPolicies APIs

Bug: 152252136
Test: Manually checked following scenario
      Step 1) Play YTM
      Step 2) Play GPM
      Step 3) Play YTM via notification panel
Change-Id: I62ce38d1c389b2a98bc536a3b232461832db3ee7