Refactor: Simplify Elasticity Manager to focus on slice availability.

This change removes all functionality related to snapshot management, resharding logic (up/down), and tracking of elastic down events or reshard retries. The class is now solely responsible for identifying and providing access to currently available ("active") slices.

Key changes include:
-   Removing methods for snapshotting, resharding, and error handling.
-   Renaming "good"/"available" slices to "active" slices for clarity.
-   Removing instance variables and initialization parameters related to snapshot periods, reshard checks, and retry limits.

PiperOrigin-RevId: 862330408

Author: lukebaumann