Change Reference.get() intrinsic to Reference.getReferent().

The reference intrinsic was incorrectly inlining
PhantomReference.get(). We now get around this by adding a
layer of indirection. Reference.get() now calls getReferent()
which is intrinsified and inlined.

Requires:
https://android-review.googlesource.com/#/c/107100/

Bug: 17429865
Change-Id: Ie91e70abf43cedf3c707c7bb8a5059e19d2a2577