Define callback for suspected data stall.

Currently, NetworkStack does not notify ConnectivityService explicitly
when it detects a potential data stall. This adds a new method
notifyDataStallSuspected() to INetworkMonitorCallbacks for notifying
ConnectivityService.

ConstantsShim is defined for using constants from the Android platform
that are introduced after the NetworkStack mainline module was defined.

Bug: 143187964
Test: compiles
Test: atest NetworkStackTests
Change-Id: Ie3b5b4e71e4a4b15a5a84f761eaead3acf6fc807
(cherry picked from commit 71aa2c64a327ee6f250a524810b0eb493148d211)