releasetools: Remove the name restriction in common.GetSparseImage.

The function used to be serving system and vendor partitions only (as
they were the only partitions using sparse image at the point). The code
itself doesn't rely on anything specific to system/vendor.

Test: python -m unittest test_common
Change-Id: Ia4ecdeedb262f3d9db082128eaf9bab299983333