[MVT Customization] Change Preferences storage key for Mobile.

MVT Custom Links / Tiles data is stored in the Preferences file under
the keys "custom_links.{initialized,list}". This is synced across
signed-in users (crrev.com/c/1263593).

Syncing can be cross-platform, i.e., Custom Links added on Desktop can
appear on Android, and vice versa. However, this might not be a good
idea since:
* Desktop Custom Tiles are all-or-nothing, whereas on Android they can
  mix with Top Sites Tiles.
* Screen real estates differ (more constrained on mobile).
* Browsing patterns across platforms may differ.

Therefore we'd like to keep {Desktop, Android} Custom Links separate.
This can be done by changing "custom_links" key to something else. More
considerations:
1. iOS Chrome does not have Custom Links yet. If the feature is added in
   the future, it makes sense to sync with Android.
2. Arguably large-form factor Android devices are Desktop-like re.
   form factor and browsing pattern?

This CL replaces "custom_links" key with a different key for Android.
Owing to (1), we choose the new key as "custom_links_mobile", and apply
it to both Android and iOS. For (2), we defer the decision and still use
"custom_links_mobile"; if the need arises we can use a separate key,
or add more data to individual custom links.

Bug: 397421764, 388782412
Change-Id: I5f9ab22fb253fab74f96b5990040010240fd9358
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6391636
Reviewed-by: Calder Kitagawa <ckitagawa@chromium.org>
Reviewed-by: Moe Ahmadi <mahmadi@chromium.org>
Commit-Queue: Samuel Huang <huangs@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1437939}