Update data type and comments on tagSocket/untagSocket APIs

- Use uint32_t for tag
- Document some possible error codes
- Explain what the tag is used for

Bug: 223423155
Test: CtsNetTestCases:android.net.TrafficStatsTest
Test: CtsNativeNetPlatformTestCases
Change-Id: Ib5b95f7d8e2158cc31183c2d5a4964244e295eb2