libstagefright: reset flag to allow buffering

When buffering is 100% or eos is reached, buffering flag
is set.If the playback is seeked back to some location
after EOS, no buffering happen. Without buffering, user experience
is bad in poor network condition.
This also handle file having header at the end. While reading
header EOS get set and buffering is prevented.

Reset the flag to allow buffering during above use cases.

CRs-Fixed: 409247

Change-Id: I60bfa4fcd0228bbb02eccb588087e9833fdf1661