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There are no longer any non-support library Alert Dialogs in
Preferences, so this theme can be removed from this activity (but is
still necessary on other activities).

The other preference-specific styles removed here were applicable only
to framework preferences and are no longer used now that all preferences
have been migrated to the support library.

Bug: 967022
Change-Id: Id367c240486b8f4bf6b892f859cd1aba5d16da73
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1711146Reviewed-by: Theresa <twellington@chromium.org>
Commit-Queue: Natalie Chouinard <chouinard@chromium.org>
Cr-Commit-Position: refs/heads/master@{#682350}