releasetools: Fix the use of StringIO.

Based on the actual semantics, it actually wants an in-memory _bytes_
buffer (io.BytesIO), especially when running with Python 3. This CL
fixes the issue and adds a unittest.

Bug: 131631303
Test: python -m unittest test_sign_target_files_apks
Test: python3 -m unittest test_sign_target_files_apks
Change-Id: I3fb067acc26713f1842e831225607779fd0d1b7e