Reland "Create an empty system_ext partition for aosp_cf_*"

This relands https://r.android.com/3304080, with the following fixes
- Skip system_ext if the product (e.g. aosp_x86_64) does not support
  system_ext
- Remove analysis error from GetFsTypeFromString if the fs type is
  unsupported. Since we are autogenerating partitions now, internal
  module creation should be skipped instead of erroring out. `fsType`
  will raise an error for handcrafted filesystem modules that explicitly
  set fs type to an unsupported type.

Test: presubmits
Test: lunch aosp_x86_64-trunk_staging-userdebug && m nothing
Change-Id: I83e0c266aa3dc2ab830c0636e1a89d00f6906968