releasetools: Add apex_utils.py.

The added `testdata/testkey_with_passwd.key` is generated with:

$ openssl pkcs8 -inform DER -in testdata/testkey_with_passwd.pk8 \
    -out testdata/testkey_with_passwd.key -passin pass:foo

Bug: 123716522
Test: python -m unittest test_apex_utils
Change-Id: I7b7f00178ecab35c3e41ecf4d10295fe9bf480ab