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/api/src/main/kotlin/io/element/android/features/call/api/CurrentCallTracker.kt

@@ -0,0 +1,23 @@
+/*
+ * 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.api
+
+/**
+ * The write side of [CurrentCallService], for whoever is actually running a call.
+ *
+ * Separate from [CurrentCallService] because the two have opposite audiences: everything in the app
+ * that wants to know whether we are in a call reads the service, while only a call implementation
+ * writes. The WebView path reaches the implementation directly through `ActiveCallManager`, both
+ * being in `features/call/impl`; the native path is in another module and would otherwise have to
+ * depend on that impl to report a call it is running itself.
+ */
+interface CurrentCallTracker {
+    fun onCallStarted(call: CurrentCall)
+
+    fun onCallEnded()
+}

features/call/api/src/main/kotlin/io/element/android/features/call/api/RingingCallTracker.kt

@@ -0,0 +1,28 @@
+/*
+ * 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.api
+
+/**
+ * Tells the ringing machinery what became of the call it is ringing for.
+ *
+ * There is one ring per incoming call, and it is owned by `ActiveCallManager` in
+ * `features/call/impl` - but *whichever* call implementation answers has to stop it, cancel the
+ * missed-call timeout and release the wake lock. The WebView path reaches the manager directly,
+ * being in the same module; the native path is in another one, and this is the narrow slice of it
+ * that anyone answering a call needs.
+ */
+interface RingingCallTracker {
+    /**
+     * The user is now in the call: stop ringing, take down the incoming-call UI, and cancel the
+     * timeout that would otherwise post a missed call.
+     */
+    suspend fun onCallJoined(callData: CallData)
+
+    /** The call is over. Clears the active call, and stops a ring that is still in progress. */
+    suspend fun onCallEnded(callData: CallData)
+}