Cleanup and refine connectivity test config

Remove the include for Connectivity/Tethering TEST_MAPPING as there is
no such file in that directory, add comments on ConnectivityModuleTest
usage, and skip the whole NetworkAgentTest on non-connectivity module
MTS runs.

Bug: 218611855
Test: atest CtsNetTestCasesLatestSdk
Change-Id: Id96ab8a5ae631b0c64f20faedd4a0430eb9318d4
3 files changed