Fix reflection error in com.google.android.datatransport.Priority when using aggressive Proguard rules. (#2016)

The enum's values are accessed via reflection, so aggressive Proguard
rules can cause it to be stripped out.
e.g., #1996

This fix switches out PriorityMapping's Map implementation to avoid reflection. 
EnumMap uses reflection under the hood. Switching to a HashMap removes the
reflection call, so we don't need the Proguard rule keeping `values`
after all.

Author: mrichards

transport/transport-runtime/src/main/java/com/google/android/datatransport/runtime/util/PriorityMapping.java

@@ -17,11 +17,11 @@
 import android.util.SparseArray;
 import androidx.annotation.NonNull;
 import com.google.android.datatransport.Priority;
-import java.util.EnumMap;
+import java.util.HashMap;
 
 public final class PriorityMapping {
   private static SparseArray<Priority> PRIORITY_MAP = new SparseArray<>();
-  private static EnumMap<Priority, Integer> PRIORITY_INT_MAP = new EnumMap<>(Priority.class);
+  private static HashMap<Priority, Integer> PRIORITY_INT_MAP = new HashMap<>();
 
   static {
     PRIORITY_INT_MAP.put(Priority.DEFAULT, 0);