Force compressing ALPHA_8 to fail

Bug: 146637821
Test: Iaf4d038379f46aa7e55384351d8098a32c4cd9ea

This failed up through O, but "succeeded" for Bitmap.CompressFormat.PNG
starting in P. The P behavior, though, uses a non-standard encoding.
When re-decoding it, Android would treat it as an all black 8888 Bitmap
with transparency, which is likely not what they wanted. Restore the
pre-P behavior.

Change-Id: I0f221fe66b6e318d2c9ed8fc199c2253f9f52497