core: Use WITH_DEXPREOPT_PIC=true to have dex2oat build pic oat files

When WITH_DEXPREOPT=true is set, if we also set WITH_DEXPREOPT_PIC=true
then the build will generate position-independent OAT images.

This means that all the system applications no longer need to be
relocated into /data/dalvik-cache and this can achieve saving disk space
at the cost of slightly slower running code.

Bug: 18035729
Change-Id: Ifd517c842f48bf31568bf5ff56667298d422c2c0