Update CS helper for clearing NetworkCapabilities UIDs.

NetworkCapabilities needs to have its UIDs cleared (UID ranges, owner
UID, and administrator UIDs) before it can be shared with apps via
ConnectivityDiagnosticsCallback invocations. The previous helper used
for clearing these values mutated the provided NetworkCapabilities. This
is updated to instead return a sanitized copy of the provided
NetworkCapabilities

Bug: 148942124
Test: atest FrameworksNetTests
Change-Id: I2431a6d273d0d73432919baf41b4f66397f4b7dc