Create implementation for fdroid flavour

app/src/fdroid/java/org/futo/circles/subscriptions/SubscriptionManagerProvider.kt

+package org.futo.circles.subscriptions
+
+import android.app.Activity
+import org.futo.circles.R
+
+object SubscriptionManagerProvider : SubscriptionProvider {
+
+    override fun getManager(
+        activity: Activity,
+        itemPurchaseListener: ItemPurchasedListener
+    ): SubscriptionManager =
+        throw IllegalStateException(activity.getString(R.string.flavour_does_not_support_subscriptions))
+}
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