fix(android): disable AudioSwitch audio focus to fix stuck volume controls after call (WT-1429)

[SERDUN]

Summary

• Disables AudioSwitch (manageAudioFocus: false) so Telecom is the sole owner of Android audio focus during VoIP calls
• Fixes volume buttons staying stuck in call mode after a call ends on older MIUI devices (Xiaomi Mi 9 Lite Android 10, Redmi Note 5 Android 9)
• One-line change in CallMediaManager._configure() — no flutter-webrtc changes needed

Root cause

Two independent audio focus clients competed for AUDIOFOCUS_GAIN:

1. Telecom (AudioFocus_For_Phone_Ring_And_Calls) — always correctly abandoned focus on call end
2. AudioSwitch (com.cloudwebrtc.webrtc.audio.c) — requested permanent AUDIOFOCUS_GAIN on call start but did not always release it

When Telecom abandoned focus, Android dispatched AUDIOFOCUS_GAIN back to AudioSwitch, which held it permanently. Android kept MODE_IN_COMMUNICATION active, so volume buttons controlled call volume instead of media/ring volume. Only Force Stop (which triggers removeFocusStackEntryOnDeath) restored normal behaviour.

Setting manageAudioFocus: false prevents AudioSwitch from requesting focus at all. Telecom becomes the sole manager and reliably releases focus on every call end. Device routing (earpiece / speakerphone / bluetooth) and MODE_IN_COMMUNICATION are unaffected — AudioSwitch still handles them regardless of the focus flag.

Test plan

• Make and end a call on Xiaomi Android 9–10 — volume buttons should control media/ring volume immediately after the call
• Verify earpiece → speakerphone toggle works during a call
• Verify Bluetooth headset routes correctly during a call
• Verify no regression on Samsung / newer Android devices
• Verify incoming call answer/reject audio routing is correct