Clear OS version and patch level fields for GKI boot images

GKI boot images will not set system version & SPL value in the
boot image header. They can be set by the device manufacturer
in the AVB properties instead.

Bug: 211741246
Test: build and `unpack_bootimg`
Change-Id: Ib357184636cb0c0c9b2a380ab3e3da7621d21790