Do not call getStagedApexInfos repeatedly

IApexService.getStagedApexInfos(session) is a costly operation. The
current implementation calls the method more than necessary.

- StagingManager calls it to get the list of apex names for each
  observer.
- Each observer calls it to get the info for each staged apex name.

In this change,, StagingManager passes the result to all observers and
each observer just use the result.

Bug: 370712193
Test: StagingManagerTest
Test: StagedInstallInternalTest
Change-Id: Ica92dbad7e72ee3c853a0fc34069c89f47f072ee