Allow using test_config_template for CTS tests.

Currently, soong will not autogenerate AndroidTest.xml files for
CTS tests. The comment says that this is because they can be used
for test data (in which case, autogenerating an AndroidTest.xml
file would not be correct).

Allow autogeneration of test files in the case that the buid rule
explicitly requests it using a test_config_template. This allows
CTS tests to use test_config_template.

Also remove a TODO that was already addressed by aosp/903934.

Test: patching aosp/1235232 and running "m CtsNetTestCases" results in test config
Change-Id: Ic3450b67c47745af5932edd82452fdf16d937ea7