BitmapDrawable: warn when passed null Bitmap

When an app passes an invalid icon into a BigPictureStyle notification,
we want to know clearly and promptly, so log when we see those icon
loads fail.

Bug: 335878768
Test: manual
Flag: NONE
Change-Id: Ic4a009f7d51ed9c40e10bb033ad0e6307a9ed3a9