Android calls: professional incoming UI + killed-state ring reliability

Addresses on-device feedback from Increment 1:
- #2 Professional incoming screen: IncomingCallActivity redesigned — brand-
  blue backdrop, circular avatar with the caller's initial, name + call
  type, and two round icon buttons (red hang-up = Decline, green phone =
  Accept) instead of the full-screen colored buttons. New vector drawables
  ic_call_answer / ic_call_decline (also used on the notification actions).
- #1 Killed-state ring: the missed-call notification arriving while killed
  proves FCM reaches the app, so the ring (high-priority DATA) is being
  held by OEM battery management (OnePlus/Oppo Doze) until the call is over.
  Added REQUEST_IGNORE_BATTERY_OPTIMIZATIONS + a requestCallReliability()
  plugin method that prompts the "allow background activity" dialog once;
  the web calls it on Android call setup. (OEM auto-launch is a manual
  toggle with no API — guide the user.)

#3 (speaker/Bluetooth) and #4 (background audio) ride Increment 2 (native
LiveKit Android media + foreground service) — kept separate so the LiveKit
dependency can't block these UI/reliability fixes (the iOS coupling lesson).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

mobile/plugins/native-call/android/src/main/res/drawable/ic_call_answer.xml

@@ -0,0 +1,9 @@
+<vector xmlns:android="http://schemas.android.com/apk/res/android"
+    android:width="26dp"
+    android:height="26dp"
+    android:viewportWidth="24"
+    android:viewportHeight="24">
+    <path
+        android:fillColor="#FFFFFF"
+        android:pathData="M6.62,10.79c1.44,2.83 3.76,5.14 6.59,6.59l2.2,-2.2c0.27,-0.27 0.67,-0.36 1.02,-0.24 1.12,0.37 2.33,0.57 3.57,0.57 0.55,0 1,0.45 1,1V20c0,0.55 -0.45,1 -1,1 -9.39,0 -17,-7.61 -17,-17 0,-0.55 0.45,-1 1,-1h3.5c0.55,0 1,0.45 1,1 0,1.25 0.2,2.45 0.57,3.57 0.11,0.35 0.03,0.74 -0.25,1.02l-2.2,2.2z" />
+</vector>

mobile/plugins/native-call/android/src/main/res/drawable/ic_call_decline.xml

@@ -0,0 +1,9 @@
+<vector xmlns:android="http://schemas.android.com/apk/res/android"
+    android:width="26dp"
+    android:height="26dp"
+    android:viewportWidth="24"
+    android:viewportHeight="24">
+    <path
+        android:fillColor="#FFFFFF"
+        android:pathData="M12,9c-1.6,0 -3.15,0.25 -4.6,0.72v3.1c0,0.39 -0.23,0.74 -0.56,0.9 -0.98,0.49 -1.87,1.12 -2.66,1.85 -0.18,0.18 -0.43,0.28 -0.7,0.28 -0.28,0 -0.53,-0.11 -0.71,-0.29L0.29,13.08c-0.18,-0.17 -0.29,-0.42 -0.29,-0.7 0,-0.28 0.11,-0.53 0.29,-0.71C3.34,8.78 7.46,7 12,7s8.66,1.78 11.71,4.67c0.18,0.18 0.29,0.43 0.29,0.71 0,0.28 -0.11,0.53 -0.29,0.71l-2.48,2.48c-0.18,0.18 -0.43,0.29 -0.71,0.29 -0.27,0 -0.52,-0.11 -0.7,-0.28 -0.79,-0.74 -1.69,-1.36 -2.67,-1.85 -0.33,-0.16 -0.56,-0.5 -0.56,-0.9v-3.1C15.15,9.25 13.6,9 12,9z" />
+</vector>