Add a CompatChange flag to enable platform backend

Apps targeting sdk < U are considered to use a legacy native
daemon as NsdManager backend, but other apps use a
platform-integration mDNS implementation as backend. So add a
CompatChange flag to enable platform backend for non-legacy
apps.

Bug: 270306772
Test: atest FrameworksNetTests CtsNetTestCases
Change-Id: I7ba58f8a5186fb49ad5f8aeacc8b8234bef1eabe
6 files changed