Refactor get partition path and IsVABC into separate function am: e012f65a1b am: 5ac9a7d20d am: 3c9703d23d am: d2a155425e

Original change: https://android-review.googlesource.com/c/platform/system/update_engine/+/1701825

Change-Id: I6b1afc3dcdf99ba78642059c5d6510da1b586bce