Correct isLegacy metric data

The isLegacy field of Nsd metrics should indicate whether the
data was collected from the old backend or not. However, it is
currently only dependent on the ENABLE_PLATFORM_MDNS_BACKEND
compat change value, which is incorrect. This is because the
NsdService always uses the new backend since Android U,
regardless of the compat change value. Therefore, the isLegacy
data should be obtained from each transaction.

Bug: 287546772
Bug: 299880473
Test: atest FrameworksNetTestCases NsdManagerTest
Change-Id: I156abd656b90578d710696a69ccf7dfca97a2c9c