Fix the media filter buffer issue in the Tuner default implementation

The current implementation does not clear the previous memcopy before
copying the next round of data into the buffer. This CL fixes the issue.

Test: atest VtsHalTvTunerV1_0TargetTest
Bug: 159027928
Change-Id: I3399c69ead046630d9c22e8fbe1e816f6ea529f9