le_audio: Do not configure device that has not contexts available

This fixes a bug that device that has no contexts available was configured
for a stream. This respects the available contexts characteristic value
exposed by the remote.

Bug: 393093038
Flag: Exempt; regression covered by unit test added
Test: atest bluetooth_le_audio_test
Change-Id: I3337b5480943af90679455f7bfd0472021c9dcf9