libstagefright: Add support to seek to zero

- Add CAN_SEEK_TO_ZERO flag in MediaExtractor to allow
  seek to zero, when previous button is pressed, for clip
  that can't be seeked.

Change-Id: Ic2761cc5bac2403ee1339da77ad02fde942a3437
Fix potential NULL pointer deference in ToneGenerator destructor

Initial members in ToneGenerator so destructor can free
allocation properly.

Change-Id: I4a686084af48f99d2eefc33e56e3239846d3d34f
Fix log spam: Failed to open libwvm.so

Bug: 7342813
Change-Id: I98852f036e4679a653723fc2bf1435d6e9d7d34c