Icon: don't create Drawables with null Bitmaps

When an app passes an invalid icon into a BigPictureStyle notification,
we want to know clearly and promptly, but to log when these icon loads
fail, we need to return null instead of wrapping a null Bitmap in a
BitmapDrawable.

Bug: 335878768
Flag: ACONFIG com.android.graphics.flags.icon_load_drawable_return_null_when_uri_decode_fails START
Test: manual
Change-Id: I460ce2c8cc1353eba3e038092d4913b75675f4fb