[Thread] fix OperationalDatasetTimestamp#toTlvValue is not loss-less

OperationalDatasetTimestamp is saving the timestamp as an Instant
object, and this has the problem that when the dataset timestamp is
written to a TLV, it's going to loss precision as the nanoseconds
of the Instant is converted to "Ticks". This results in bug that
the dataset timestamp doesn't equal to the same value when it's
converted back from a TLV:

```
var timestamp1 = OperationalDatasetTimestamp.fromInstant(Instant.ofEpochSecond(100, 999938900));
byte[] tlv = timestamp1.toTlvValue();
timestamp2 = OperationalDatasetTimestamp.fromTlvValue(tlv);

assertThat(timestamp2).isEqualTo(timestamp1); // This fails!!!
```

Bug: 308408536
Test: atest ThreadNetworkUnitTests CtsThreadNetworkTestCases
Change-Id: I81b482a05337b6d1150edaa28b860034592a4347