Element Call ringing notifications (#2978)

- Add `ActiveCallManager` to handle incoming and ongoing calls.
- Add ringing call notifications with full screen intents and missed call ones as part of the 'conversation' notifications.

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Co-authored-by: ElementBot <benoitm+elementbot@element.io>

appconfig/src/main/kotlin/io/element/android/appconfig/ElementCallConfig.kt

@@ -17,5 +17,13 @@
 package io.element.android.appconfig
 
 object ElementCallConfig {
+    /**
+     * The default base URL for the Element Call service.
+     */
     const val DEFAULT_BASE_URL = "https://call.element.io"
+
+    /**
+     * The default duration of a ringing call in seconds before it's automatically dismissed.
+     */
+    const val RINGING_CALL_DURATION_SECONDS = 15
 }