Fixed id referencing in WebVTT CSS styling

In CSS, ids are references using #. The absence of # references elements.

NOTE: If the id of a cue was "1", we support its reference with ::cue(#1).
In CSS, however, this is not valid, and the number should be escaped with
\3 as in ::cue(\31). We still do not use number escaping (and I doubt
whether we should at some point).
-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=119634708

library/src/androidTest/assets/webvtt/with_css_styles

@@ -10,14 +10,14 @@ STYLE
 NOTE comment blocks can be used between style blocks.
 
 STYLE
-::cue(2) {
+::cue(#id2) {
   color: peachpuff;
 }
 
-1
+id1
 00:00.000 --> 00:01.234
 This is the first subtitle.
 
-2
+id2
 00:02.345 --> 00:03.456
 This is the second subtitle.

library/src/main/java/com/google/android/exoplayer/text/webvtt/WebvttCue.java

@@ -41,7 +41,8 @@ import java.util.Map;
 /* package */ final class WebvttCue extends Cue {
 
   public static final String UNIVERSAL_CUE_ID = "";
-  
+  public static final String CUE_ID_PREFIX = "#";
+
   public final String id;
   public final long startTime;
   public final long endTime;
@@ -122,7 +123,7 @@ import java.util.Map;
     public WebvttCue build(Map<String, WebvttCssStyle> styleMap) {
       // TODO: Add support for inner spans.
       maybeApplyStyleToText(styleMap.get(UNIVERSAL_CUE_ID), 0, text.length());
-      maybeApplyStyleToText(styleMap.get(id), 0, text.length());
+      maybeApplyStyleToText(styleMap.get(CUE_ID_PREFIX + id), 0, text.length());
       if (position != Cue.DIMEN_UNSET && positionAnchor == Cue.TYPE_UNSET) {
         derivePositionAnchorFromAlignment();
       }