Include ares_android.(c|h) in the build config.

This change adds `ares_android.h` and `ares_android.c` to the `third_party/cares/cares.BUILD` in order to support builds in the Android Native Development Kit (NDK).

Fixes grpc#21437

Author: damonkey2

third_party/cares/cares.BUILD

@@ -113,6 +113,7 @@ cc_library(
         "ares__get_hostent.c",
         "ares__read_line.c",
         "ares__timeval.c",
+        "ares_android.c",
         "ares_cancel.c",
         "ares_create_query.c",
         "ares_data.c",
@@ -162,6 +163,7 @@ cc_library(
     ],
     hdrs = [
         "ares.h",
+        "ares_android.h",
         "ares_build.h",
         "ares_config.h",
         "ares_data.h",