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This switches Dev UI from Activities to Fragments. The immediate user
benefit is smoother transitions when switching between tools, but this
opens up for opportunities for other navigation methods (ex. bottom nav
bar or nav drawer).

Bug: 1017532
Test: Tested system_webview_apk on L+, monochrome on N+, Trichrome on Q
Test: Click flag UI notification, verify it opens the flags UI fragment
Change-Id: I32eb7a9b2ada6395bad21488e6bd15c802131b7e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2108919
Commit-Queue: Nate Fischer <ntfschr@chromium.org>
Reviewed-by: Hazem Ashmawy <hazems@chromium.org>
Cr-Commit-Position: refs/heads/master@{#753124}