Add U to GetDexVersion

Change-Id: I93f501bfb756ede33bd70a0c5e8e68b09a6e4096
diff --git a/src/main/java/com/android/tools/r8/utils/DexVersion.java b/src/main/java/com/android/tools/r8/utils/DexVersion.java
index 443b9e7..d094cf7 100644
--- a/src/main/java/com/android/tools/r8/utils/DexVersion.java
+++ b/src/main/java/com/android/tools/r8/utils/DexVersion.java
@@ -42,6 +42,7 @@
         // version.
       case ANDROID_PLATFORM:
       case MASTER:
+      case U:
       case T:
       case Sv2:
       case S: