Add test for VMDebug.isDebuggerConnected behavior around DDMS.

VMDebug.isDebuggerConnected has the somewhat strange behavior of not
considering a debugger connected until it has issued at least one
non-ddms call. Some debugger clients rely on this behavior.

We add an explicit test that JDWP connections that have only been used
for DDMS traffic are not considered 'connected'.

Test: ./art/tools/run-jdwp-tests.sh --mode=host
Test: ./art/tools/run-libjdwp-tests.sh --mode=host

Bug: 70527014
Bug: 69169846
Change-Id: Ib4a40fdabe329122ced49cc0e76917a30aec490f