Enable error-prone checking for CtsNetTestCasesDefaults

Enable error-prone checking for CtsNetTestCasesDefaults to
catch NullablePrimitive errors during build time. This is
to address review feedback at aosp/3292201

Test: m CtsNetTestCases
Bug: N/A
Change-Id: I009f0e4ede02a933570cfcc8c563808dbf436193
1 file changed