Remove incorrect @NonNull annotations on getDrawable() and variants.

For compatibility, had to continue returning null when drawables could
not be decoded. Fix annotation to match pre-P behavior (the behavior
was reverted separately).

Fixes: 69543526
Test: make

Partial revert of Ib01eca970c5c9969998ce5b265b120aa7048b41a

Change-Id: I5f612f47793c3f04cf9874e13efdc13397ddd4e8