ANDROID: GKI targets sets common_kernel.defconfig.

... instead of DEFCONFIG in build configs. This is
a step towards deprecating build configs.

I did not delete DEFCONFIG from build.config.gki because
it is still sourced downstream.

Also sets check_defconfig_minimized that corresponds to
POST_DEFCONFIG_CMDS=check_defconfig.

Bug: 368119551
Change-Id: Ia5208a1758667223141573c91fbe3a0d20d7c19e
Signed-off-by: HONG Yifan <elsk@google.com>