Build Support for CTS Native Tests

Bug 4970300

Adds two new variables, CTS_TEST_CASES and CTS_TEST_XMLS, to be read
from CtsTestCaseList.mk. The CTS_TEST_CASES variable can be used to
copy any sort of file to the repository/testcases CTS directory.
The CTS_TEST_XMLS variable can be used to inject test package xmls
from any source rather than relying upon the monolithic and
mostly serial buildCts.py script.

The existing CTS_CORE_CASE_LIST is coded to only support APKs, so
it could not be retrofitted to support native tests. However, the
two new variables can do even more than CTS_CORE_CASE_LIST due to
their generality. In the future, the idea is move away from
CTS_CORE_CASE_LIST and also generate XMLs using separate tools
rather than just buildCts.py.

Change-Id: Ib52722861c37e0f4d511f9041928395bcaba5dea