Make OnHeaderDecodedListener NonNull

Bug: 73788928
Test: No change in behavior; no new tests.

There is already an overload for both decodeDrawable and decodeBitmap
that allows not using an OnHeaderDecodedListener, so require calls to
the method that includes a parameter for it to contain a non-null
version.

Throw an exception if OnHeaderDecodedListener is null. Move
implementation to Impl methods to share code without lint warnings.
Rename the non-static decodeBitmap to decodeBitmapInternal, to
distinguish it from the others.

Change-Id: I12c0a35786d8d1267044ac09b5c9203cdfb7175a