sounds_index: Avoid repeatedly reindexing.

The sounds index is rebuilt each time a format is registered or
unregistered.  This causes the index to be repeatedly rebuilt during
startup and shutdown.

This patch significantly reduces the work done by delaying sound index
initialization until after modules are loaded.  This way a reindex only
occurs if a format module is loaded after startup.  We also skip
reindexing when format modules are unloaded during shutdown.

Change-Id: I585fd6ee04200612ab1490dc804f76805f89cf0a