Use IBinder as key for ConnectivityDiagnostics storage in CS.

This change updates ConnectivityService to use IBinder instances as keys
when storing ConnectivityDiagnosticsCallbacks.

When storing ConnectivityDiagnosticsCallbacks in ConnectivityService,
the IConnectivityDiagnsoticsCallback is used as the key for
ConnectivityService.mConnectivityDiagnosticsCallbacks. However,
IConnectivityDiagnosticsCallback instances are received as different
objects. This causes them to produce different hashCode() values, so
attempts to remove an IConnectivityDiagnosticsCallback fail.

Bug: 150867635
Test: atest FrameworksNetTests
Change-Id: Ib99e68d5ae47fa27e12428f9a60a2c1204ac59a2