Aggregate prepared statement cache stats from individual connections.

Previously, prepared statement cache statistics were tracked globally within `SQLiteConnectionPool`. This change moves the tracking to individual `SQLiteConnection` instances and aggregates these statistics when `collectDbStats` is called on the pool. This provides more accurate per-connection stats and a better overall picture of cache usage across all connections in the pool.

This also corrects the definition of prepared statement cache.

Bug: 440440268
Flag: EXEMPT refactor
Change-Id: Ic2a399a8d033a5a9ca1a02a7ab9ce0598ac44ae8