[DocsUI] Rebase PickerPreviewAllTypeUiTest on ActivityTestJunit4

The current PickerPreviewAllTypeUiTest is based on ActivityTest which
precludes the use of @RequiresFlagsEnabled etc for controlling test
execution.

Flag: EXEMPT bugfix
Bug: 430421551
Test: atest DocumentsUIGoogleTests:com.android.documentsui.PickerPreviewAllTypeUiTest
Change-Id: Ib370679ad7d05be4e093b342de00556f241333c3