[Mahi] Deprecate MahiPrefsController

Also:
1) Renamed `kMahiEnabled` to `kHmrEnabled ` (b/346684308)
2) Used the new MagicBoostState in MagicBoostOptInCard and
MagicBoostCardController (b/341158134)
3) Fix the mojom sequence issue (b/346622428)

Bugs: b/346684440, b/346684308, b/341158134, b/346622428
Change-Id: Ia25e0f0edfd3e676b56d927471c0a5834709f5b2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5630263
Commit-Queue: Jiaming Cheng <jiamingc@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Emily Stark <estark@chromium.org>
Reviewed-by: Andre Le <leandre@chromium.org>
Reviewed-by: Alex Ilin <alexilin@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1316607}