Bug 5819 - Violation of synchronization rules in AbstractSchemaRepository

As noted by @GuardedBy annotation, access to sources field need to be
performed while holding the object monitor. Refactor getSchemaSource()
to create a copy under lock and then release it for further processing.

Change-Id: I65fddf5830e657c3601c85fd976f12e3c8bb3ff6
Signed-off-by: Alexis de Talhouët <adetalhouet@inocybe.com>
Signed-off-by: Robert Varga <nite@hq.sk>
(cherry picked from commit 36bdb3a032d4924b36dc86d782e58043954adadc)