Report bitmap config when decoding only metadata

With the introduction of support for 16 bit images, bitmaps returned
by BitmapFactory may be using the RGBA_F16 configuration. Some apps
and libraries assume the returned configuration is always ARGB_8888
and make assumptions based on that to compute cache sizes, etc.

This changes extract the output config when BitmapFactory is invoked
with the inJustDecodeBounds option. Despite its name, this option
is already used (and documented!) to decode more than bounds as it
also decodes the MIME type.

Bug: 35108998
Test: CtsGraphicsTestCases
Change-Id: If71959751458816678e42b21ab26c889aba5dea0