recovery: Add runtime checks for A/B vs traditional updates

This allows A/B devices to install legacy (non-payload) style update.zip
packages like our addonsu or gapps.

Do not abort an update if META-INF/com/android/metadata is missing,
instead use it to determine whether the update is an A/B ota or not.

Change-Id: I907a92732470b947de007dde11fb71db64d94c9b