Allow more characters in subtypes

Allow all printable ASCII characters in subtypes, instead of only the
set of characters allowed in service types.

This is still more restrictive than what the spec allows, but allowing
random bytes would be harder to manage (especially when displaying the
subtypes in logs for example) and there is no known use-case for it.
Having the NsdManager API impose more restrictions is probably also best
for apps to minimize interoperability problems.

Bug: 324377460
Test: atest
Change-Id: Id75879a1c3ea6af8d7995b0e8e0dcc06e7160a0a