Revert "[DIPS] Rename DIPS prefix to Btm."

This reverts commit 4d4b33ce5cf2e18c6f2b2a120b2a8064db75c351.

Reason for revert: Tree closure for compile failure

Original change's description:
> [DIPS] Rename DIPS prefix to Btm.
>
> This makes it match the external name (Bounce Tracking Mitigations) and
> comply with the C++ style guide, which says "prefer to capitalize
> abbreviations as single words".
>
> A followup CL will rename filename prefixes from dips_ to btm_ (and the
> directories from dips/ to btm/).
>
> Bug: 388538934
> Change-Id: I43d3f1c24632919ac8985aea5b7f2d945d2a9628
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6172859
> Owners-Override: Avi Drissman <avi@chromium.org>
> Commit-Queue: Ryan Tarpine <rtarpine@chromium.org>
> Reviewed-by: Avi Drissman <avi@chromium.org>
> Cr-Commit-Position: refs/heads/main@{#1406975}

Bug: 388538934
Change-Id: Ic86e19c9c3a23d7dcb014bab5bc6bc13419a0ae0
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6173773
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Auto-Submit: Joey Arhar <jarhar@chromium.org>
Owners-Override: Joey Arhar <jarhar@google.com>
Commit-Queue: Joey Arhar <jarhar@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1406979}