Add Class.getPrimitiveClass().

Implement the native part in ART and remove the native parts
of the previously needed workaround for java.lang.Void.TYPE.

Companion libcore/ change:
    https://android-review.googlesource.com/619326

Test: m test-art-host-gtest
Test: testrunner.py --host --optimizing
Test: Pixel 2 XL boots.
Bug: 73346078
Change-Id: Icfb201a1f08d8ff2a65c58f7c0ec25aa2296db29