ART: Added benchmark for string dex cache

Added a benchmark for the string dex cache, measures how long the fast
path takes. In order to account for potential optimizations, I have
separated it into two different functions, one of which is called from a
loop so the code is no longer in the loop. Please subtract ~1.5ns from
the runtime per iteration as a result.

Change-Id: I77803ebf83b18b806288349e1e5b3d26ac215d84
1 file changed