Read compat config from APEX

Expose getActiveApexInfos for CompatConfig to use, then go over it
searching for configs in etc/compatconfig.

Test: Put a change in an APEX, flash device, inspect
`adb shell dumpsys platform_compat` output. It contains the change.
Bug: 142650523
Bug: 138222363

Change-Id: I9e249900271eb1ea6d34a1ca02b8ba1dd8eddb6b