Fix Sonar Issues in sli/core module

Fix Sonar Issues in sli/core module

Few major issues
*Remove unused imports
To increase code readability
*Replace the type specification with the diamond operator
To reduce the verbosity of generics code
*Add the "@Override" annotation above this method signature
To increase code readability
*Remove this unnecessary cast to "String"
Unnecessary casting expressions pollutes code

Issue-Id: CCSDK-87
Change-Id: I38952f5026a5c61af73ac16706e8ddc278566b13
Signed-off-by: surya-huawei <a.u.surya@huawei.com>