Invoke dex2oat explictly before dalvikvm

The goal is to stop relying on dex2oat being run when dex files are
loaded.

Test: ./out/host/linux-x86/bin/art --profile -Ximage:$PWD/out/host/linux-x86/framework/core.art -Xusejit:true -Xcompiler-option --compiler-filter=speed -Xcompiler-option --compiler-backend=Optimizing -verbose:oat -Djava.library.path=$PWD/out/x86_64/host/linux-x86/lib64 -cp $PWD/generated/benchmarks.dex benchmarks.ExoPlayerBench.java.ExoPlayerBench

Bug: 36824842
Change-Id: I9f4a1384cdc91502edea08402ee984c57ff8c37d