Separate libdexfile external C ABI from the C++ header file.

Also add a test to compile it with C.

Test: m
Test: m test-art-{host,target}-gtest-dex_file_ext_c_test
Test: art/test/testrunner/run_build_test_target.py -j80 art-test
Bug: 120978655
Change-Id: I6e9944a2051bef9a2775bc2072df4e0ed81dc833
diff --git a/build/Android.gtest.mk b/build/Android.gtest.mk
index a926d9a..d95777f 100644
--- a/build/Android.gtest.mk
+++ b/build/Android.gtest.mk
@@ -383,6 +383,7 @@
     art_hiddenapi_tests \
     art_imgdiag_tests \
     art_libartbase_tests \
+    art_libdexfile_external_tests \
     art_libdexfile_tests \
     art_libprofile_tests \
     art_oatdump_tests \