[dexopt] Allow secondary dex files to generate app images

After creating tests and fixing bugs surrounding loading multiple app
images into the runtime we can safely enable generating app image files
for secondary dexes.

Test: adb shell pm bg-dexopt-job with app installed that uses secondary
dexes, verify app image is generated for secondary image. Launch app and
verify no class linker/class loader/image space warnings in logcat
Bug: 149098478

Change-Id: Iaa04175ee56aae69f41f3e2972c322049cfc9e26