REG: Android: Fix freeze when accessing assets

Change 287fa94fe2f93e2857a4c15f69435c4ea14de82e created a freeze
in the assets file engine because it will try locking the mutex
twice. Since prepopulateCache() is only called from create(),
we don't need to lock it recursively.

Task-number: QTBUG-37661
Change-Id: I00d0fed132a86c1be5603484eb6ee05454da9ef0
Reviewed-by: BogDan Vatra <bogdan@kde.org>