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
Bug: 113175337

Change-Id: Ia4ecdeedb262f3d9db082128eaf9bab299983333
Merged-In: Ia4ecdeedb262f3d9db082128eaf9bab299983333