Enrich in-flight theme downloads for Synced Set Up Undo/Redo

When a theme collection wallpaper is still downloading via
NtpSyncedThemeManager during Synced Set Up import,
settingsToApply.getTheme() initially has a null bitmap. Once the
download finishes, construct an enriched
NtpBackgroundDataThemeCollection with the downloaded bitmap and local
file metadata while preserving the remote platformType and any explicit
primaryColor, and reset isBitmapSaved(false) in applyThemeSettings() so
Redo re-saves bitmap files deleted during Undo. Also call
maybeApplyBackgroundUpdateFromDeviceSync() when applying a null-bitmap
Android theme so cached synced themes apply on non-NTP pages.

Bug: 517615321
Change-Id: I127f8a201875478baaab88614626de29ae83e87b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/8436698
Commit-Queue: Jenna Himawan <jhimawan@google.com>
Reviewed-by: Mark Schillaci <mschillaci@google.com>
Reviewed-by: Xi Han <hanxi@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1704651}