Move dexpreopt image selection out of dexpreopt package

Instead of passing both the module and global dexpreopt image
selection into the dexpreopt package, determine the correct
dexpreopt image in the java package.

Also stop using the boot image "location" as an input, only
track the real path, and then convert it to the "location"
that dex2oat expects when constructing the command line.

Test: m checkbuild
Change-Id: I2be2b5917e09fd57428dab27849c0153fdc75722