Tweak temporary RON permission UI to make it clearer.

Hide dismiss button for now; add explanatory text (same as Settings uses);
add background to distinguish from importance levels.

Test: atest SystemUITests
Flag: com.android.systemui.permission_helper_ui_rich_ongoing
Change-Id: I68fbdfb1b0a856dbe7fc808c3e95a3ca5f7ad01e