[docsui] More robust root selection in sidebar in tests

1. Skip clicking on a root in the sidebar if the target root is already
   selected.

   UiObject.click() waits for a TYPE_WINDOW_CONTENT_CHANGED or
   TYPE_VIEW_SELECTED accessibility event, and will return false if a
   timeout (3s by default) is reached without an event happening. If the
   target root is already selected, clicking it doesn't appear to
   generate these events, failing the test.

2. Since we are only clicking if the root isn't already selected,
   the drawer should be closed automatically, so stop closing it
   manually.

3. Wait for the title to change after clicking.

4. Make checks in assertWindowTitle more specific.

   Only check header_title for pre-m3 recents root, because checking
   header_title substring for other roots is not a reliable indicator
   that the root is selected, as the title will contain the root's name
   even if a subdirectory is selected.

This also rolls back ag/34396707, which isn't needed anymore.

Bug: 428680975
Test: atest DocumentsUIGoogleTests:com.android.documentsui.CancelFromNotificationUiTest
Flag: EXEMPT test change
Change-Id: I6b780d674ed8f6c293dfbbb87c50e74bfbaf4ca7