Call clearCallingIdentity before notifyDnsResponse

The NetworkStack only expects calls from UID 1000 (system_server) or the
Bluetooth app. onDnsEvent is triggered by Netd which has UID 0.

One alternative would be to allow UID 0 to call the NetworkStack
directly, but being more restrictive on callers sounds like a better
option.

Test: Flashed, booted, atest FrameworksNetTests
Change-Id: Id7fb30f1e25ec70fbfbc90f3c7fc95ba18c274e6