releasetools: Clean up sign_target_files_apks.py.

Mostly cosmetic changes, such as replacing print statement with print
function. Also change 'import cStringIO' to optionally look for the one
in io module, to allow Python 2/3 compatibility.

Test: pylint --rcfile=pylintrc sign_target_files_apks.py
Test: Run sign_target_files_apks.py on marlin target_files.zip.
Change-Id: I4dc98b01da6f89e624114bbca5522f659901c1f2