Forward unknown Data Stall types to Connectivity Diagnostics.

This CL forwards suspected Data Stall events detected with unknown
detection methods to ConnectivityDiagnostics.

Currently, ConnectivityService drops any data stall events with unknown
detection methods, which leads to false negatives for Connectivity
Diagnostics registrants. This change ensures that registrants will still
be notified as NetworkStack is updated to use new detection methods.

The documentation for ConnectivityDiagnosticsManager#DataStallReport is
also updated to reflect that the detection methods included in the
report are a bit mask of detection methods used. Implicitly, this means
that data stalls detected via unknown methods will have an empty bit
mask (0x00).

Bug: 156294356
Test: atest ConnectivityDiagnosticsManager
Change-Id: I62d0bf91fcc17c7921afd519c72551399906bd6b