Open up new API in StagingManager to get information about staged APEX

These APIs will be later used by PackageManagerNative service to open up
the information to native clients.

The current implementation is a bit wasteful since everytime the client
makes a request, we ask the ApexManager to fetch us information about
the staged sessions N times, where N is the number of staged sessions
that are marked ready.

We can optimize it by caching the result of getStagedApexInfos(session)
method. This will be done in a separate CL.

Bug: 187444679
Test: atest StagingManagerTest
Change-Id: I76803382db26a029f6d9cabb0d9ce9a2bf9c8ea1