The test data is generated as follows:

$ mkdir dir
$ base64 /dev/urandom | head -c 100000 > dir/file1
$ base64 /dev/urandom | head -c 100000 > dir/file2
$ soong_zip -o test.zip -D dir

Running zipinfo shows that the files are compressed using deflate (defN)
$ zipinfo test.zip
Archive:  test.zip
Zip file size: 151594 bytes, number of entries: 2
-rw-------  2.0 unx   100000 bl defN 08-Jan-01 00:00 dir/file1
-rw-------  2.0 unx   100000 bl defN 08-Jan-01 00:00 dir/file2
2 files, 200000 bytes uncompressed, 151352 bytes compressed:  24.3%