[Feature sync] fix handling arbitrary bytes in TXT value

RFC 6763 defines that TXT value can accept both utf-8 string and
binary data. Current implementation will always cast the TXT
value to a utf-8 string and will cause data lose when there are
non-utf-8 chars in the TXT value. This commit fixes this by
having the browser passing the TXT values back as byte[].

Also fixed the TXT key&value parsing issues per RFC 6763
section 6.5: accept cases of no '=' and reject empty key.

Bug: 254155029
Test: atest FrameworksNetTests CtsNetTestCases
Change-Id: I4b755e60ad6e59db19faa41556dd214993d73896