Simplify staged install resume flow.

In short, new flow looks like this:

1. Check state of apex sessions.
2. If it's inconsistent (e.g has activated and failed sessions) -> abort
3. If there is a failed apex session, fail all remaining sessions.
4. Otherwise proceed with resuming sessions one-by-one.

Test: atest CtsStagedInstallHostTestCases
Test: atest ApexManagerTest
Test: atest StagingManagerTest
Bug: 176452293
Change-Id: Ide45ee9977b291a4ce2ca3d1cdebf53f464891f8