IMediaPlayer.cpp: make sure structures are initialized to 0

Credit https://code.google.com/p/android/issues/detail?id=183310

Bug: 23515142
Change-Id: Idbd66fb148bd0ac1dd78f8651d0164f2a41e2427
(cherry picked from commit b73b826cc16291b33649402497efbe0f946413bd)