Fix apexer and apexer_test

Apexer should use apex_build_info.pb for various arguments. But
--payload_fs_type was missed. Besides, --payload_fs_type having a default
value(ext4) is problematic when using apex_build_info. It prevents the
argument from being copied from apex_build_info when it's missing from
CLI (because it's considered as given always).

For apexer_test,
- adding fsck.erofs as executable
- removing redundant test code (the test is to verify
  --unsigned_payload_only, but it did some extra check even after
  --unsigned_payload_only is verified to produce the desired output.)

Bug: 381321189
Test: apexer_test
Change-Id: I727d29804347f9bbddfcd95e567d6f12a7d2f1c8
2 files changed