updater: add functions to modify dynamic partition metadata

Test: sideload full OTA on cuttlefish
Test: sideload incremental OTA on cuttlefish (that grows
      system, shrinks vendor, and move vendor to group foo)
Test: verify that /cache/recovery/cc46ebfd04058569d0c6c1431c6af6c1328458e4
      exists (sha1sum of "system")

Bug: 111801737

Change-Id: Ibdf6565bc1b60f3665c01739b4c95a85f0261ae5