Add OWNERS for VectorDrawableTest

Without this, changes to these files require approval from top level
OWNERS. These are tests for the code sitting under
android.graphics.VectorDrawable, so use the graphics OWNERS. Model the
file on core/tests/coretests/src/android/graphics/OWNERS.

Bug: NA
Test: NA
Change-Id: I6feefca0abef7b699b5fa544d652fc528065a7bb