Use boot image extension in the JIT-zygote experiment.

Test: temporarily enable JIT-zygote configuration, build, boot the
  device and ensure that the JIT-zygote specific boot image
  apex-framework.art is mapped in the zygote address space:

  1. enable Jit zygote in the product device config (in my case
    device/google/muskie/aosp_walleye.mk):

    +# System server should not contain compiled code.
    +PRODUCT_SYSTEM_SERVER_COMPILER_FILTER := verify
    +
    +# Use the apex image for preopting.
    +DEXPREOPT_USE_APEX_IMAGE := true
    +
    +# Have the runtime pick up the apex image.
    +PRODUCT_PROPERTY_OVERRIDES += \
    +    dalvik.vm.boot-image=/apex/com.android.art/javalib/apex.art:/system/framework/apex-framework.art

  2. lunch aosp_walleye-userdebug \
    && m \
    && adb reboot bootloader \
    && fastboot flashall -w

  3. adb shell cat /proc/`adb shell ps | grep zygote64 | awk {'print $2'}`/maps | grep apex-framework.art
  6fe44000-7025c000 rw-p 00000000 fc:02 1179718                            /data/dalvik-cache/arm64/apex@com.android.art@javalib@apex-framework.art
  70571000-70696000 rw-p 0072d000 fc:02 1179718                            /data/dalvik-cache/arm64/apex@com.android.art@javalib@apex-framework.art
  75a339f000-75a33ac000 r--p 00852000 fc:02 1179718                        /data/dalvik-cache/arm64/apex@com.android.art@javalib@apex-framework.art

Change-Id: I5493e575ebf90bad1d5ad2850004d54590bbc079