IDB: Rename databbase_impl.h/.cc to indexed_db_connection.h/.cc

This is part two of a change that merged IndexedDBConnection and
DatabaseImpl into a single class.

See part one here:
https://chromium-review.googlesource.com/c/chromium/src/+/5345854

Bug: 41483624
Change-Id: Ib3fa6844898543decf937c69635ff7083435f577
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5355814


Reviewed-by: Daseul Lee <dslee@chromium.org>
Reviewed-by: Evan Stade <estade@chromium.org>
Commit-Queue: Steve Becker <stevebe@microsoft.com>
Reviewed-by: Joshua Bell <jsbell@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1270539}