Fix silence-injection policy skip in VirtualAudioController

Unconditionally register mAudioPlaybackDetector in startListening to ensure the deferred silence injection policy update recovers correctly. Conditionally register mAudioRecordingDetector only when configChangedCallback is provided.

Bug: 515034370
Bug: 521392631
Test: atest VirtualAudioControllerTest
Flag: EXEMPT BUGFIX
Cherrypick-From: https://googleplex-android-review.googlesource.com/q/commit:43c51e370480ac0404b71962514cb76682c5fb1d
Merged-In: I2af61fc32fb2d19e68037fdb8a10a23dc4b37071
Change-Id: I2af61fc32fb2d19e68037fdb8a10a23dc4b37071