releasetools: Fix an issue in common.GetAvbPartitionArg.

It's a bug introduced in commit 1aeef725, which affects unittest only.

Bug: 130351427
Test: Use a lunch'd target. Run
      `atest --host releasetools_test releasetools_py3_test`.
Change-Id: I7ff01a6af47d002e1203bd376d477b60d769cbd1