Allow OTA package to contain fake AVB version

The system_matrix.xml in compatibility.zip can contain a fake
AVB version as specified by BOARD_OTA_FRAMEWORK_VBMETA_VERSION_OVERRIDE.
This allows OTA to pass compatibility check when the AVB version is
updated *during* the OTA.
Bug: 112291283
Test: specify BOARD_OTA_FRAMEWORK_VBMETA_VERSION_OVERRIDE and
      manual test

Change-Id: I53c2de49ee68d6a3afc97201e759be344de6bdff