[FIX] ListBinding: adapted visibility of getContexts
[FIX] ListBinding: adapted visibility of getContexts

Due to several problems caused by side effects of the getContexts
function its visibility has to be changed. The getCurrentContexts method 
is publicly available and should satisfy the same requirements. 

Change-Id: Iaae7bdc5ae74595f78f2cd594e427c791fa7dba8

SAP/openui5,SAP/openui5,SAP/openui5,olirogers/openui5,cschuff/openui5,olirogers/openui5,SQCLabs/openui5,SQCLabs/openui5,olirogers/openui5,nzamani/openui5,nzamani/openui5,SQCLabs/openui5,SAP/openui5,cschuff/openui5,nzamani/openui5,cschuff/openui5