Apply global audio options at the engine level

Introduce ApplyGlobalOptions to the MediaEngine interface to configure
global audio processing options (such as AEC, AGC, and noise
suppression) directly at the engine level. This replaces the previous
behavior where media channels implicitly re-applied options to the
engine during initialization and parameter updates.

Applying global settings during CreateAudioSource ensures engine-level
configurations persist reliably across stream lifecycles. This change
also separates global audio processing options from stream-level
parameters like jitter buffer and audio network adaptor settings.

                 [Before Decoupling]
+----------------------------------------------------+
|      AudioOptions (AEC/AGC/NS + Jitter/ANA)        |
+----------------------------------------------------+
  /                                                \
 / (engine()->ApplyOptions)                         \
v                                                    v
WebRtcVoiceEngine (Global APM)             VoiceSend/RecvChannel


              [After Decoupling (This CL)]
+-----------------------+                  +-----------------------+
|  Global Options (APM) |                  |  Channel Options (ST) |
|  - echo_cancellation  |                  |  - jitter_buffer      |
|  - auto_gain_control  |                  |  - audio_network_     |
|  - noise_suppression  |                  |    adaptor            |
+-----------------------+                  +-----------------------+
           |                                           |
           v (ApplyGlobalOptions)                      v
   WebRtcVoiceEngine (APM)                     VoiceSend/RecvChannel
   - Stores in global_options_                 - No ApplyOptions calls
   - Persisted across cycles                   - No leakage / racing

Bug: webrtc:42224170
Change-Id: I3c23293ae338495ee32330ce5287411947f6d472
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/476340
Reviewed-by: Per Åhgren <peah@webrtc.org>
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47844}