Add JVMTI DDMS extension method and event.

Add a new jvmti extension method
'com.android.art.internal.ddm.process_chunk' that can be used to
request that the system process a DDMS chunk with a given type and
data payload. It returns the processed chunk type and data. Agents can
use this to interact with DDMS.

Also add a new jvmti extension event
'com.android.art.internal.ddm.publish_chunk' that will be called
whenever the system wishes to send an unrequested chunk of data to be
processed. This is triggered by code executing 'DdmServer#sendChunk'
or by other internal mechanisms.

Both of these extensions are provided mainly to aid in the maintenence
of backwards compatibility with existing DDMS applications.  Generally
agents should prefer to use the normal JVMTI events and controls over
interpreting DDMS data or calling DDMS functions.

Bug: 62821960
Test: ./test.py --host -j50
Test: ./art/tools/run-jdwp-tests.sh --mode=host \
            --test org.apache.harmony.jpda.tests.jdwp.DDM.DDMTest
Change-Id: I39f22d3d096d12b59713ec7b8b0c08d0d68ff422