[Audiosharing] Gate legacy audio sharing

1. Avoid open broadcast dialog via intent when flag is off.
2. Move to settingslib flag to use one flag to gate legacy UI in both
   SysUI and Settings.

Bug: 322295262
Flag: ACONFIG com.android.settingslib.flags.enable_le_audio_sharing DEVELOPMENT
Test: atest
Change-Id: If075620b69d066bb1ef126b869982d29ec8bf0c6