BigPictureStyle: discard empty Icon BitmapDrawables

If Icon.loadDrawable successfully opens an InputStream for an Icon's
URI, but fails to decode an image from that stream, it currently returns
a BitmapDrawable with a null Bitmap.

Right now, we treat that as a valid drawable and try to render it,
leaving a blank space in the notification where the bitmap would've gone.

Instead, when the corresponding flag is enabled, treat these empty
BitmapDrawables as if loadDrawable had returned null directly.

TODO: I dunno why but my flag isn't showing up when I build this change?

Bug: 335878768
Flag: ACONFIG android.widget.flags.big_picture_style_discard_empty_icon_bitmap_drawables START
Test: manual
Change-Id: I7adda22d97072e7d234c2c58501ad6b2334db997