Updates to handler logic for multilayer requests

Updates to ConnectivityService network request handler's logic to support
(or disallow support of) multilayer network requests.

Bug: 175239920
Bug: 171991028
Test: atest FrameworksNetTests
atest NetworkStackTests
atest FrameworksNetIntegrationTests
atest NetworkStackIntegrationTests
atest CtsNetTestCasesLatestSdk

Change-Id: Ic67cff950d72745d6508a0a037bd33f932d5132c