Add sign_apex.py that signs a given APEX file.

This CL moves SignApex() from sign_target_files_apks into apex_utils,
and adds sign_apex that allows signing a standalone APEX file directly.

Test: Run the following command and check the output file.
$ build/make/tools/releasetools/sign_apex.py \
     -v \
     --container_key \
         build/make/target/product/security/testkey.x509.pem \
     --payload_key external/avb/test/data/testkey_rsa4096.pem \
     --payload_extra_args \
         "--signing_helper_with_files ./signing-helper.sh" \
     foo.apex \
     signed-foo.apex
Test: Run sign_target_files_apks.py on crosshatch target_files.zip.
Change-Id: I4b2422fd5cb1c60a3aa94511475e2a0e5b1666ca