dex_cache: rewrite StringDexCachePair as DexCachePair<T>

.. with [T = mirror::String]. This is in preparation for introducing
a dex cache array for MethodTypes, which will be treated the same way.

Test: make test-art-host
bug: 30550796
Change-Id: Ief4455b4c6e4c9dd897f2c40b14b843a57b1dc8e
6 files changed