stagefright: set scaling mode for blank frames in ACodec

Analogous to 1d5ac80d
(by jgennis), this sets the scaling mode for the ANativeWindow
to SCALE_TO_WINDOW prior to pushing the blank frames during
decoder tear down.

Without this, the window defaults to FREEZE and SF ignores the
new frames.

Bug: 9516405
Change-Id: I39ef30922d733034bf01100d7ff24ac9c0c33b7d
Signed-off-by: Dima Zavin <dima@android.com>