frameworks/base: Fix for no sound output issue in preview ringtone.

        - When you repeatedly adjust volume up/down in preview ringtone,
          sound output is lost. Mediaplayer instaces are created and are
          not released correcly which causes this issue. Fixed the issue
          by adding a check before creating any new Mediaplayer instance.

Change-Id: I794d29839501a93d0a42463435e50078fea93ab5
CRs-fixed: 245668