releasetools: Add a testcase for common.ParseCertificate().

Also minor clean up to the function, to suppress the following lint
warning.
R:1813, 2: Redefinition of cert type from list to unicode (redefined-variable-type)

Test: python -m unittest -v test_common.CommonApkUtilsTest
Change-Id: Ib4b1dfa8b19e505fc70684b648efc36171c73bbf