Root cause: MainActivity set MODE_IN_COMMUNICATION at app launch,
hijacking system audio routing immediately — BT A2DP music dropped to
earpiece, and the pre-existing communication mode confused subsequent
setCommunicationDevice calls for BT SCO.
Fix: MainActivity now only sets volumes. MODE_IN_COMMUNICATION is set
via JNI right before Oboe audio_start() in CallEngine, and MODE_NORMAL
is restored after audio_stop() when the call ends.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>