Don't override keep-annotation system property if set

Change-Id: I5d0b519050fad581ce15e84854e6280a55272ef3
diff --git a/src/main/java/com/android/tools/r8/R8Command.java b/src/main/java/com/android/tools/r8/R8Command.java
index 035f730..2fc05e8 100644
--- a/src/main/java/com/android/tools/r8/R8Command.java
+++ b/src/main/java/com/android/tools/r8/R8Command.java
@@ -1158,7 +1158,9 @@
 
     assert !internal.enableTreeShakingOfLibraryMethodOverrides;
 
-    internal.testing.enableEmbeddedKeepAnnotations = enableExperimentalKeepAnnotations;
+    if (enableExperimentalKeepAnnotations) {
+      internal.testing.enableEmbeddedKeepAnnotations = true;
+    }
 
     if (!internal.isShrinking()) {
       // If R8 is not shrinking, there is no point in running various optimizations since the