Move the *ts-current-api generation to it's own Makefile.

Currently building cts-current-api will give it the
CtsDevicePolicyManagerTestCases config file. This causes the test
to run twice.

This fix moves the definition to it's own Android.mk in a subdirectory.

Bug: 131758938
Test: make cts-current-api;
      verify that android-cts/testcases/current.api was built;
      checked that cts-current-api.config was not built.

Change-Id: I4327829bb2815787c176acaf59d66ad39e872787