[DocsUI M3] Update Nav rail UI test

* The device screen size check should happen before launching
  the activity.
* Use standard way to convert between px and dp.
* Add comments for some methods in SidebarBot to make it more
  explicit.

Bug: 410436015
Test: atest DocumentsUIGoogleTests:com.android.documentsui.NavRailUiTest
Flag: com.android.documentsui.flags.use_material3
Change-Id: I7df6a8d021902da103731430cc9023dd4f625e74