ANDROID: cuttlefish_defconfig: Fix dm-verity related options

Use the suggestions mentioned in the APEX documentation
https://source.android.com/devices/tech/ota/apex



Also, enable DM_VERITY_AVB on x86, which was incorrectly disabled.

Change-Id: Id9e1b0bf525802dd67a9bf4b7c6c6408e87948fd
Signed-off-by: Alistair Delva <adelva@google.com>