Add functions to query submission queue capacity am: 301d17205a

Original change: https://android-review.googlesource.com/c/platform/system/update_engine/+/2308914

Change-Id: I4826140bccd0130b647c746eab45e2a50769b332
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>