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: I5fdab94da352e8014225b2454aacc2a5b5fc6a52

adb/apex/Android.bp

@@ -21,7 +21,7 @@ apex {
 }
 
 // adbd apex with INT_MAX version code, to allow for upgrade/rollback testing.
-apex {
+apex_test {
     name: "test_com.android.adbd",
     defaults: ["com.android.adbd-defaults"],
     manifest: "test_apex_manifest.json",