[FIX] sap.ui.rta: fixed Utils.getIndex for singular aggregations
[FIX] sap.ui.rta: fixed Utils.getIndex for singular aggregations

Change-Id: I24c06859c3066c374a05dce4bf5a97737c2259ba
Signed-off-by: Kevin Edinger <2cd308f6e534f22a23b481bd971d726fce0a3425@sap.com>

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