Ensure that a PushProvider is available on a device before using it.

It help to fallback to UnifiedPush (if available) if the PlayServices are not installed on the device.

libraries/push/api/src/main/kotlin/io/element/android/libraries/push/api/PushService.kt

@@ -24,6 +24,10 @@ interface PushService {
     // TODO Move away
     fun notificationStyleChanged()
 
+    /**
+     * Return the list of push providers, available at compile time, and
+     * available at runtime, sorted by index.
+     */
     fun getAvailablePushProviders(): List<PushProvider>
 
     /**