Define Bundle keys for ConnectivityDiagnostics.

ConnectivityDiagnosticsManager defines several callbacks that can be
invoked for registered ConnectivityDiagnosticsCallback instances.
onConnectivityReport and onDataStallSuspected include ConnectivityReport
and DataStallReport, respectively, both of which contain a
PersistableBundle to be used for passing information from the
NetworkStack module to the registered callback. When values will be
consistently passed inside these bundles, their keys are defined inside
ConnectivityReport and DataStallReport.

Bug: 147249364
Test: compiles
Test: atest FrameworksNetTests
Change-Id: Icc6bd4337865692b6ca7c18eacabff59ba3a5bb9