Lazily create schema context in GlobalBundleScanning*

Lazily create schema context in GlobalBundleScanning*

On OsgiBundleScanningSchemaService startup, it calls
tryToUpdateSchemaContext when bundleTracker.open() completes.
This parses and creates a schema context and notifies listeners
of the updated schema context. However we don't have to call
tryToUpdateSchemaContext in start() unless there actually are
listeners registered as the purpose of tryToUpdateSchemaContext
is to update listeners. So made this change. It's not likely there
would be any listeners at that point anway.

I also changed the synchronization so as not to synchronize on 'this'
or the class instance as both are unsafe (users could sync on the
instance causing unwanted contention, either innocently or maliciously).

Change-Id: I435358b0851671b7fbfdc9784577c91ff20556df
Signed-off-by: Tom Pantelis <tpanteli@brocade.com>
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>