Consistently use either "boot" or "apex" boot image as the default.

Previous CL Ia9b34aa92ebb1b4de96ea0f8f290d798be19b2cf introduced
asymmetry in handling "boot" and "apex" boot images: in JIT-zygote
experiment, though the "apex" boot image was installed, the "boot"
boot image was still used for generating dexpreopt configs.

It is unclear why this asymmetry was needed at that point; it seems
incorrect to use different boot images for dexpreopting and for
installing on device. After recent changes the asymmetry started
breaking walleye_jitzygote-userdebug on git_rvc-release, because APK
dexpreopt commands refer to inexistent boot image files.

Test: lunch aosp_walleye-userdebug && m
Test: cherry-pick CL in internal, walleye_jitzygote-userdebug boots

Change-Id: Id877c10269cf79caf6ae74b1dc169a31e6a2211b