Pass correct --image argument to oatdump.

This was broken after CL:
  Ida40dfae8c83bf7c2e737d5c7ea418e1197ad826
..which split boot image into primary and extension.
The argument to oatdump was not updated properly.

The old form before the split was:

  --image=out/soong/generic_x86/dex_bootjars/system/framework/boot.art

The new form after the split is:

  --image=out/soong/generic_x86/dex_artjars/apex/com.android.art/javalib/boot.art:out/soong/generic_x86/dex_bootjars/system/framework/boot-framework.art

Test: lunch aosp_x86-userdebug && m dump-oat

Test: Manually inspect build command to make sure correct image
  location is used:

  $ fgrep -e 'oatdump' $ANDROID_BUILD_TOP/out/soong/build.ninja | egrep -e '--image[^ ]*' -o
  --image=out/soong/generic_x86/dex_artjars/apex/com.android.art/javalib/boot.art:out/soong/generic_x86/dex_bootjars/system/framework/boot-framework.art

Change-Id: I54a5146c404aefb6eaeadbe3e9363db935ecad86