Default SpatializationBehavior to Auto and make Never opt-in

[robertwu1]

Motivation

Previously, Oboe explicitly overrode SpatializationBehavior::Unspecified to SpatializationBehavior::Never by default (added in commit 9e24a77022d20cf89c4a444873e8a82b72781b6d, PR #1776) with a comment // Override Unspecified as Never to reduce latency, and also defaulted mSpatializationBehavior to Never on API <= 31 and OpenSL ES.

However, overriding Unspecified to Never prevents Android XR from spatializing 2D panel app audio for apps using Oboe. When AAudioStreamBuilder_setSpatializationBehavior is not called (or set to AAUDIO_UNSPECIFIED), AAudio's native default on API 32+ is AAUDIO_SPATIALIZATION_BEHAVIOR_AUTO (and on API <= 31 / OpenSL ES, the system uses its default automatic behavior).

Therefore, SpatializationBehavior::Auto should be the default everywhere (mirroring the system's default behavior), and SpatializationBehavior::Never should be opt-in (supported on AAudio API 32+).

Empirical Latency & MMAP Verification

We ran an empirical stream-open matrix on an attached Pixel 10 (frankel, Android 17 / API 37) comparing SpatializationBehavior::Auto vs SpatializationBehavior::Never across PerformanceMode (LowLatency and None), SharingMode (Shared and Exclusive), and channel counts (2 Stereo and 6 5.1 Surround):

PerformanceMode	SharingMode	Channels	SpatializationBehavior	Actual Perf	Actual Sharing	MMAP Used	Frames / Burst	Buffer Capacity
LowLatency	Shared	2 (Stereo)	Auto	LowLatency	Shared	Yes	96 (2ms)	1536
LowLatency	Shared	2 (Stereo)	Never	LowLatency	Shared	Yes	96 (2ms)	1536
LowLatency	Exclusive	2 (Stereo)	Auto	LowLatency	Exclusive	Yes	96 (2ms)	1920
LowLatency	Exclusive	2 (Stereo)	Never	LowLatency	Exclusive	Yes	96 (2ms)	1920
LowLatency	Shared / Exclusive	6 (5.1)	Auto	None	Shared	No	962	1924
LowLatency	Shared / Exclusive	6 (5.1)	Never	None	Shared	No	962	1924
None	Shared / Exclusive	2 or 6	Auto	None	Shared	No	962	2886 / 1924
None	Shared / Exclusive	2 or 6	Never	None	Shared	No	962	2886 / 1924

• Findings: On Android phones, setting SpatializationBehavior::Auto has zero impact on low-latency stereo streams (AUDIO_OUTPUT_FLAG_MMAP_NOIRQ / AUDIO_OUTPUT_FLAG_FAST are preserved with identical 96-frame bursts and MMAP paths), while enabling Android XR to spatialize 2D panel app audio automatically.

Changes

• src/aaudio/AudioStreamAAudio.cpp: Default SpatializationBehavior::Unspecified to SpatializationBehavior::Auto (and set mSpatializationBehavior = SpatializationBehavior::Auto when builder_setSpatializationBehavior == nullptr on API <= 31).
• src/opensles/AudioStreamOpenSLES.cpp: Set mSpatializationBehavior = SpatializationBehavior::Auto since OpenSL ES does not support opting out via Never and always uses the system's default behavior.
• include/oboe/AudioStreamBuilder.h: Update documentation comment for setSpatializationBehavior to state that SpatializationBehavior::Auto is the default and SpatializationBehavior::Never is only supported on API 32 and above for AAudio.
• tests/testStreamOpen.cpp: Update spatialization unit tests (AAudioInputSetSpatializationBehavior, AAudioOutputSetSpatializationBehavior, OpenSLESOutputSetSpatializationBehavior, AAudioInputSetSpatializationBehaviorUnspecified, and AAudioOutputSetSpatializationBehaviorUnspecified) to test that Auto is the default everywhere and Never is opt-in on API 32+ AAudio.

Testing

• Verified all 5 spatialization tests in testStreamOpen.cpp and the full 242-test GoogleTest suite on Pixel 10 (frankel, Android 17 / API 37).