Add tests for sockoptTimeVal functions

Prepare to add Os.getSockOptTimeVal() and Os.setSockOptTimeVal()
to the public SDK API surface along with StructTimeVal. This will also
enable them to be removed from the core-platform and intra-core APIs.

Test: atest CtsLibcoreTestCases:libcore.android.system.StructTimevalTest
Test: atest CtsLibcoreTestCases:libcore.libcore.io.OsTest
Bug: 129251251
Bug: 117867568
Bug: 129433363
Change-Id: Ie0b629195f1aac50d78b61203d2f7b91be3454c9