Fix --boot-image argument in dex2oat command for boot image extension.

The primary image name should be "boot.art" in case of
"boot-framework.art", and "apex.art" in case of "apex-framework.art".
Although "boot.art" and "apex.art" are identical binaries, dex2oat
has hard-coded logic based on the image name.

This has been broken since boot image extension has been enabled for
JIT-zygote config in CL I5493e575ebf90bad1d5ad2850004d54590bbc079.

Test: compare boot-framework.art and apex-framework.art,
  they were identical before this CL, and differ after this CL.

Test: JIT-zygote config boots, steps 1-2:

  1. Temporarily enable Jit zygote in the product device config (in
    this 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. Build and flash:
    $ lunch aosp_walleye-userdebug && m \
    && adb reboot bootloader && fastboot flashall -w

Change-Id: I98de271852ecc33feb9fd4c9b0addf0feba01856