Android: protect global jni cache.

This fixes a issue that has been neglected for a while, namely, that
the access to the global jni caches where not sufficiently protected
for concurrent usage. This change also fixes an issue with the
thread-name storage.

Task-number: QTBUG-42755
Change-Id: I22f95ae7f44d1f6a13e289e52b050d98ccb9fb28
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@theqtcompany.com>