Use UTF-8 everywhere

UTF-8 is the default charset on Android so this should be a no-op change, but
makes the code portable (in case it runs on another platform).

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=167011583

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

@@ -15,6 +15,7 @@
  */
 package com.google.android.exoplayer2.util;
 
+import com.google.android.exoplayer2.C;
 import java.nio.ByteBuffer;
 import java.nio.charset.Charset;
 
@@ -428,7 +429,7 @@ public final class ParsableByteArray {
    * @return The string encoded by the bytes.
    */
   public String readString(int length) {
-    return readString(length, Charset.defaultCharset());
+    return readString(length, Charset.forName(C.UTF8_NAME));
   }
 
   /**

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

@@ -253,7 +253,7 @@ public final class Util {
    * @return The code points encoding using UTF-8.
    */
   public static byte[] getUtf8Bytes(String value) {
-    return value.getBytes(Charset.defaultCharset()); // UTF-8 is the default on Android.
+    return value.getBytes(Charset.forName(C.UTF8_NAME));
   }
 
   /**