Make Os.get/setSockOptTimeVal() public

Add Os.getSockOptTimeVal() and Os.setSockOptTimeVal() to the public
API surface along with StructTimeVal. This enables 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: Ib67cdc1b47f14397866babcf65c10c71ab6d0039