[mdns] add API for discovery with subtype

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