Skip the test if no mainline modules installed

The test failed because we failed to enable rollback for
ModuleMetadataGoogle which is not a module on AOSP builds.

Let's skip the test for builds which have no mainline modules
installed.

Bug: 152721401
Test: atest StagedRollbackTest
Change-Id: Iaf75dea5388e2a751a26f983a79924c4358d4833

tests/RollbackTest/RollbackTest/src/com/android/tests/rollback/StagedRollbackTest.java

@@ -477,4 +477,12 @@ public class StagedRollbackTest {
         StorageManager sm = (StorageManager) context.getSystemService(Context.STORAGE_SERVICE);
         assertThat(sm.isCheckpointSupported()).isTrue();
     }
+
+    @Test
+    public void hasMainlineModule() throws Exception {
+        String pkgName = getModuleMetadataPackageName();
+        boolean existed =  InstrumentationRegistry.getInstrumentation().getContext()
+                .getPackageManager().getModuleInfo(pkgName, 0) != null;
+        assertThat(existed).isTrue();
+    }
 }

tests/RollbackTest/StagedRollbackTest/src/com/android/tests/rollback/host/StagedRollbackTest.java

@@ -243,6 +243,7 @@ public class StagedRollbackTest extends BaseHostJUnit4Test {
      */
     @Test
     public void testRollbackWhitelistedApp() throws Exception {
+        assumeTrue(hasMainlineModule());
         runPhase("testRollbackWhitelistedApp_Phase1");
         getDevice().reboot();
         runPhase("testRollbackWhitelistedApp_Phase2");
@@ -460,4 +461,16 @@ public class StagedRollbackTest extends BaseHostJUnit4Test {
             return false;
         }
     }
+
+    /**
+     * True if this build has mainline modules installed.
+     */
+    private boolean hasMainlineModule() throws Exception {
+        try {
+            runPhase("hasMainlineModule");
+            return true;
+        } catch (AssertionError ignore) {
+            return false;
+        }
+    }
 }