Disable auto-dismiss of visual signals

The current auto-dismiss timing is too short and makes it difficult to
interact with the signals. Disable it for now so we can see how
intrusive the signals are and as a stopgap measure for now.

Bug: 439695733
Test: atest 'DocumentsUIGoogleTests:com.android.documentsui.JobPanelViewModelTest'
Test: atest 'DocumentsUIGoogleTests:com.android.documentsui.JobPanelUiTest'
Flag: com.android.documentsui.flags.visual_signals_ro
Change-Id: I96400e386b29acbd504382e4a49e3d4d7b454a47