android/ui: navigate back home after switching users

Updates tailscale/corp#18202

Signed-off-by: Percy Wegmann <percy@tailscale.com>

android/src/main/java/com/tailscale/ipn/MainActivity.kt

@@ -131,7 +131,11 @@ class MainActivity : ComponentActivity() {
           composable("about") { AboutView(nav = backNav) }
           composable("mdmSettings") { MDMSettingsDebugView(nav = backNav) }
           composable("managedBy") { ManagedByView(nav = backNav) }
-          composable("userSwitcher") { UserSwitcherView(nav = backNav) }
+          composable("userSwitcher") {
+            UserSwitcherView(
+                nav = backNav,
+                onNavigateHome = { navController.popBackStack(route = "main", inclusive = false) })
+          }
           composable("permissions") {
             PermissionsView(nav = backNav, openApplicationSettings = ::openApplicationSettings)
           }

android/src/main/java/com/tailscale/ipn/ui/view/UserSwitcherView.kt

@@ -25,7 +25,11 @@ import com.tailscale.ipn.ui.viewModel.UserSwitcherViewModel
 
 @OptIn(ExperimentalMaterial3Api::class)
 @Composable
-fun UserSwitcherView(nav: BackNavigation, viewModel: UserSwitcherViewModel = viewModel()) {
+fun UserSwitcherView(
+    nav: BackNavigation,
+    onNavigateHome: () -> Unit,
+    viewModel: UserSwitcherViewModel = viewModel()
+) {
 
   val users = viewModel.loginProfiles.collectAsState().value
   val currentUser = viewModel.loggedInUser.collectAsState().value
@@ -64,6 +68,8 @@ fun UserSwitcherView(nav: BackNavigation, viewModel: UserSwitcherViewModel = vie
                         if (it.isFailure) {
                           viewModel.showDialog.set(ErrorDialogType.LOGOUT_FAILED)
                           nextUserId.value = null
+                        } else {
+                          onNavigateHome()
                         }
                       }
                     })