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: Ie7dc9ad1444904f49047093a44e7fcd0a5f56ffe

apex/permission/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.permission",
     visibility: [
         "//system/apex/tests",

apex/statsd/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.os.statsd",
     visibility: [
         "//system/apex/tests",