Tweak download notifications

1. Add string for STATE_CANCELED. Lint doesn't like that the
   switch statement on the state IntDef doesn't have a case
   for STATE_CANCELED. May as well add one, even if we're not
   planning on our demo app showing notifications for this
   state.
2. Replace non-human-readable error message with one provided
   by ErrorMessageProvider.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=184120892

library/core/src/main/java/com/google/android/exoplayer2/util/ErrorMessageProvider.java

@@ -17,17 +17,15 @@ package com.google.android.exoplayer2.util;
 
 import android.util.Pair;
 
-/**
- * Converts exceptions into error codes and user readable error messages.
- */
-public interface ErrorMessageProvider<T extends Exception> {
+/** Converts throwables into error codes and user readable error messages. */
+public interface ErrorMessageProvider<T extends Throwable> {
 
   /**
    * Returns a pair consisting of an error code and a user readable error message for the given
-   * exception.
+   * throwable.
    *
-   * @param exception The exception for which an error code and message should be generated.
+   * @param throwable The throwable for which an error code and message should be generated.
+   * @return A pair consisting of an error code and a user readable error message.
    */
-  Pair<Integer, String> getErrorMessage(T exception);
-
+  Pair<Integer, String> getErrorMessage(T throwable);
 }

testutils/build.gradle

@@ -24,9 +24,9 @@ android {
     }
 
     lintOptions {
-        // Truth depends on JUnit, which depends on java.lang.management, which is not part of
-        // Android. Remove this when JUnit 4.13 or later is used. See:
-        // https://github.com/junit-team/junit4/pull/1187.
+        // Truth depends on JUnit, which depends on java.lang.management, which
+        // is not part of Android. Remove this when JUnit 4.13 or later is used.
+        // See: https://github.com/junit-team/junit4/pull/1187.
         disable 'InvalidPackage'
     }
 }