Create an empty system_ext partition for aosp_cf_*

Cuttlefish uses erofs, which is now supported. With this CL, empty
system and system_ext filesystem soong modules will be created

This CL also makes ext4 the default fs. If
`BOARD_$PARTITION_IMAGE_FILE_SYSTEM_TYPE` is empty, ext4 will be used.

Bug: 372487849
Test: lunch aosp_cf_x86_64_phone-trunk_staging-userdebug && m nothing
Test: verified that soong modules are created for system and system_ext
Change-Id: Ib4eecb63e86a09a45853279c1afce5c26e5202d4