mobile UI: don't exit when using back button to close drawers

An Android user might reasonably assume that they can use the back button to
close the global or context drawers. So act accordingly.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>

mobile-widgets/qml/main.qml

@@ -880,4 +880,14 @@ if you have network connectivity and want to sync your data to cloud storage."),
 		manager.finishSetup()
 		manager.appInitialized()
 	}
+	onClosing: {
+		if (globalDrawer.visible) {
+			globalDrawer.close()
+			close.accepted = false
+		}
+		if (contextDrawer.visible) {
+			contextDrawer.close()
+			close.accepted = false
+		}
+	}
 }