Update exoplayer to r1.5.6
Update exoplayer to r1.5.6

Change-Id: I1ed12b7f9b9208f3fd0ae076bcfa5098327785ed

googlearchive/androidtv-Leanback,googlesamples/androidtv-Leanback,KazumiHARADA/TvCleanArchitecture2,cartland/androidtv-Leanback