Comment reason for simulateDataStall structure.

This change adds a comment to CS#simulateDataStall to explain why the
Data Stall is wrapped in a DataStallReportParcelable before being passed
to the ConnectivityDiagnostics handler. This approach is taken to ensure
that simulated data stalls are handled the exact same as Data Stalls
received directly from NetworkMonitor (including Data Stalls detected by
methods that the platform does not understand).

Bug: 156294356
Test: atest ConnectivityDiagnosticsManager
Change-Id: I751054418bf328c72b977a1cc99c27cb9b8ab7ba