ART: Change StringTable to unordered_set

Cache the string hash in the table element. Trade space for time.

Effect on b/37620770 repro case: Dump time = 3.7s -> 1.6s

(cherry picked from commit 84eadb2d)

Bug: 37620770
Test: m test-art-host
Merged-In: Ifaed1c65e30097851188450a5db2bc8ee30efbb4
Change-Id: Ifaed1c65e30097851188450a5db2bc8ee30efbb4