Add minimal dex set for hiddenapi

hiddenapi, as invoked when the dist target is added, requires that the
boot classpath be nonempty.

Since we want to run dist on the buildbots, I have added a minimal group
of boot jars to the classpath and bumped the image size to fit them.

Bug: 122330825
Test: make PRODUCT-qemu_trusty_arm64-userdebug DIST_DIR=something dist
Change-Id: I8b93f8c54a8a491f13654a531b26ec938ba38933