This commit adds support of discovering services with explicit subtype.
With this change, a service can be discovered with subtype with:
```
nsdManager.discoverServices(
new DiscoveryRequest.Builder("_http._tcp", PROTOCOL_DNS_SD)
.setSubtype("_printer").build(),
executor, listener);
```
Bug: 265095929
Test: atest CtsNetTestCases FrameworksNetTests
Change-Id: Iba76283a003cf2d52a8c26e1de872c3e8e433350
97d6aeb3a2