[Audiosharing] Show loading state on audio sharing page

1. Show "Sharing audio stream..." once starting audio sharing
2. Show "Sharing with <devicename>..." once adding source to the device

Test: atest
Flag: com.android.settingslib.flags.enable_le_audio_sharing
Bug: 362858894
Change-Id: I6c19d999baaa91d6a5365f24e88efe79c2b38072