Fix community search issues

[micahmo]

Pull Request Description

This PR includes a few changes for the community search page.

1. Focus the search box immediately. Unlike the main search page, there is no other content here, so I think it makes sense to jump right to searching.
2. Fix an issue with search state being lost when navigating back via the Android gesture.
   • I discovered that, when using the Android back button, the SwipeablePageRoute will rebuild itself, causing the blocs that it creates to be reconstructed from scratch. The fix is simply to construct the bloc outside of the SwipeablePageRoute. (This could possibly help with solving #1109 as well!).

Issue Being Fixed

Issue Number: N/A

Screenshots / Recordings

Issue 1

focus_search.mp4

Issue 2 Before

state_issue_before.mp4

Issue 2 After

state_issue_after.mp4

Checklist

• Did you update CHANGELOG.md?
• Did you use localized strings where applicable?
• Did you add semanticLabels where applicable for accessibility?

[hjiangsu]

LGTM!