use apex_test for test apexes

... so that the build system can distinguish testing APEXes and installable APEXes, and apply
different rules to them.

Bug: 146308764
Test: m
Change-Id: I6ffd1cfe0f83c032d6fe3fa11e38c3c6c763425a

apex/sdkextensions/testing/Android.bp

@@ -12,7 +12,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-apex {
+apex_test {
     name: "test_com.android.sdkext",
     visibility: [ "//system/apex/tests" ],
     defaults: ["com.android.sdkext-defaults"],