Navigate to Install unknown apps in SPA

Guarded by feature flag SETTINGS_ENABLE_SPA, default false.
When flag enabled, will navigate to the new "Install unknown apps" page
which is implemented by SPA.

Bug: 235727273
Test: Manual test with Settings app
Change-Id: Ife038e4b2b23520f6cc46075a6bae6282f11f1af

src/com/android/settings/spa/SpaActivity.kt

@@ -16,6 +16,19 @@
 
 package com.android.settings.spa
 
+import android.content.Context
+import android.content.Intent
 import com.android.settingslib.spa.framework.BrowseActivity
 
-class SpaActivity : BrowseActivity(settingsPageProviders)
+class SpaActivity : BrowseActivity(settingsPageProviders) {
+    companion object {
+        @JvmStatic
+        fun startSpaActivity(context: Context, startDestination: String) {
+            val intent = Intent(context, SpaActivity::class.java).apply {
+                putExtra(KEY_DESTINATION, startDestination)
+            }
+            context.startActivity(intent)
+        }
+    }
+}
+