ART: Remove old aget on null workaround

Use null for an aget-object of null. Ensure that other aget types
on null cannot be converted to or used as a reference type.

Let the verifier continue scanning after an aget on a null register,
to ensure that the dead code is type-safe.

Add test coverage for the new behavior.

Partially reverts commit 4824c279
Partially reverts commit 857f058d

Bug: 22059710
Bug: 64683522
Bug: 69669661
Test: m test-art-host
Change-Id: Ie0b554e8f880251d8e73ab6dfb6b41a5e63defc6