With this change, also refactored runtest as follows: Modified the
test suite schema and python implementation to have an inheritance
structure.  Each test type has its own python module, which will
also handle the logic of running the test.

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.

Improved error handling, and added support for "size" and "package" arguments.
Removed deprecated runtest shell script.