Avoid using dex cache array pointers in libart.

Avoid using dex cache array pointers in libart.

In preparation for making dex cache arrays native, avoid
using them in Java code.

This causes a performance regression for our reflection
benchmarks. Class_getDeclaredMethod and Class_getMethod
take an up to 30% hit, measured using the Quick compiler.
We accept this hit at this stage and we will tune the
performance after we're done with the larger effort.

Companion libcore/ change:
    https://android-review.googlesource.com/146069

Bug: 20134538
Change-Id: Ibbef3b50043a1311cd40723ed42e1f1c609b8fc1