leaudio: Add audio server scanner to the LeAudioService

Whenever there are LeAudio devices bonded, make sure to have scanner
enabled in order to make targeted announcements filtering work,
especially when screen is off.

Bug: 275753723
Bug: 276656074
Test: manual
Tag: #feature
Change-Id: I37dca9676efa2caee4f915a0e39fe07847fc20d9