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
Merged-In: Ie3b5b4e71e4a4b15a5a84f761eaead3acf6fc807

• apishim/29/com/android/networkstack/apishim/api29/ConstantsShim.java[Added - diff]
• apishim/30/com/android/networkstack/apishim/ConstantsShim.java[Added - diff]
• common/networkstackclient/src/android/net/INetworkMonitorCallbacks.aidl[diff]
• src/com/android/server/connectivity/NetworkMonitor.java[diff]
• tests/unit/src/com/android/server/connectivity/NetworkMonitorTest.java[diff]