Android: Don't crash on 100+ combo box items

Local references are valid until the original native method
returns (if it's a Java thread), or until the current thread
is detached from the VM. This means that we have to manually
manage local references and delete them when they are no longer
in use, otherwise we will leak memory and flood JNI's local
reference array.

[ChangeLog][Android] Fixed crash on populating large combo boxes
or menus.

Task-number: QTBUG-36074
Change-Id: I7095bd1472f759be98183737a8d3912fa9763f30
Reviewed-by: Christian Stromme <christian.stromme@digia.com>