Expose the write sides of the current-call and ringing state

`CurrentCallService` and `ActiveCallManager` are read by the whole app but
written only by whoever is actually running a call. Writing has so far been
reachable only from inside `features/call/impl`, because that is where both
implementations live.

Adds `CurrentCallTracker` and `RingingCallTracker` to `features/call/api`, so
that a call implementation in another module can report the call it is running
and answer a ring, without depending on `features/call/impl`.

No behaviour change: both interfaces are implemented by the existing classes,
and their methods are aliases of methods that already existed.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

features/call/test/src/main/kotlin/io/element/android/features/call/test/FakeCurrentCallTracker.kt

@@ -0,0 +1,24 @@
+/*
+ * Copyright (c) 2026 Element Creations Ltd.
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial.
+ * Please see LICENSE files in the repository root for full details.
+ */
+
+package io.element.android.features.call.test
+
+import io.element.android.features.call.api.CurrentCall
+import io.element.android.features.call.api.CurrentCallTracker
+
+class FakeCurrentCallTracker : CurrentCallTracker {
+    /** Every transition in order, so a test can assert the call was reported before it was cleared. */
+    val calls = mutableListOf<CurrentCall>()
+
+    override fun onCallStarted(call: CurrentCall) {
+        calls += call
+    }
+
+    override fun onCallEnded() {
+        calls += CurrentCall.None
+    }
+}

features/call/test/src/main/kotlin/io/element/android/features/call/test/FakeRingingCallTracker.kt

@@ -0,0 +1,24 @@
+/*
+ * Copyright (c) 2026 Element Creations Ltd.
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial.
+ * Please see LICENSE files in the repository root for full details.
+ */
+
+package io.element.android.features.call.test
+
+import io.element.android.features.call.api.CallData
+import io.element.android.features.call.api.RingingCallTracker
+
+class FakeRingingCallTracker : RingingCallTracker {
+    val joined = mutableListOf<CallData>()
+    val ended = mutableListOf<CallData>()
+
+    override suspend fun onCallJoined(callData: CallData) {
+        joined += callData
+    }
+
+    override suspend fun onCallEnded(callData: CallData) {
+        ended += callData
+    }
+}