Adds cts test definitions to the testrunner/test_defs.xml.
Adds support for runtest --cts arg, which will run all cts tests.
This temporarily relies on the addition of a 'cts' attribute to the test
definition - a new xml format may be defined later that changes how cts tests
are identified.
This change is based on previous unsubmitted CL
https://android-git.corp.google.com/g/Gerrit#change,1702. The only delta wrt
to that change is the use of InstrumentationCtsTestRunner.
Update: rebased to latest donut.
