[android][test] Use Android SDK in swift -frontend

`test/lit.cfg` correctly specifies the `-sdk` flag on Darwin,
but does not do so on Linux or Android. Set the flag on Android
for now.

Author: modocache

test/lit.cfg

@@ -722,8 +722,8 @@ elif run_os == 'linux-androideabi':
         % (config.swiftc, config.variant_triple, resource_dir_opt, mcp_opt,
            config.swift_test_options, swift_execution_tests_extra_flags))
     config.target_swift_frontend = (
-        '%s -frontend -target %s %s'
-        % (config.swift, config.variant_triple, resource_dir_opt))
+        '%s -frontend -target %s -sdk %s %s'
+        % (config.swift, config.variant_triple, config.variant_sdk, resource_dir_opt))
     subst_target_swift_frontend_mock_sdk = config.target_swift_frontend
     subst_target_swift_frontend_mock_sdk_after = ""
     config.target_run = config.test_source_root + \