Add in-app sponsor support

app/src/main/java/io/nekohasekai/sfa/ui/main/SettingsFragment.kt

@@ -103,6 +103,11 @@ class SettingsFragment : Fragment() {
         binding.openDebugButton.setOnClickListener {
             startActivity(Intent(requireContext(), DebugActivity::class.java))
         }
+        binding.startSponserButton.setOnClickListener {
+            Vendor.startSponsor(requireActivity()) {
+                activity.launchCustomTab("https://sekai.icu/sponsor/")
+            }
+        }
         lifecycleScope.launch(Dispatchers.IO) {
             reloadSettings()
         }

app/src/main/java/io/nekohasekai/sfa/vendor/VendorInterface.kt

@@ -5,5 +5,5 @@ import android.app.Activity
 interface VendorInterface {
     fun checkUpdateAvailable(): Boolean
     fun checkUpdate(activity: Activity, byUser: Boolean)
-
+    fun startSponsor(activity: Activity, fallback: () -> Unit)
 }