generate multiple boot.img files in aosp_arm64

Generate boot.img variants with different schemes for kernel compression
(gzip, lz4, none).

The boot image is removed from vbmeta for aosp_arm64 since it is
not possible currently to store the hashes of multiple variants of a
particular image. The GKI will be verified via VTS anyway.

Also remove some emulator-specific stuff from aosp_arm64.

Bug: 151094943
Change-Id: I465ea6c98d52e839284d51b246203f6b8dfb5566