Frameworks/base: Support dex2oat threads system property

Check dalvik.vm.dex2oat-threads in AndroidRuntime and pass to ART
with "-j" as a compiler option, if found.

Check dalvik.vm.image-dex2oat-threads in AndroidRuntime and pass to
ART with "-j" as an image compiler option, if found.

Bug: 19992386

(cherry picked from commit e0352388)

Change-Id: I5e7806cf560607d31a1d6901dffb14bee538c9cc