Format: Move language into common fields section

Language is already applicable to image formats as well.
It can also apply to video formats. For example when the
video contains burnt in subtitles. Also, some animation
studios produce multiple variants of animations, in which
text within the scene itself (e.g., a signpost) is
replaced to be language specific.

PiperOrigin-RevId: 294500227

library/common/src/test/java/com/google/android/exoplayer2/FormatTest.java

@@ -66,6 +66,7 @@ public final class FormatTest {
         new Format(
             "id",
             "label",
+            "language",
             C.SELECTION_FLAG_DEFAULT,
             C.ROLE_FLAG_MAIN,
             /* bitrate= */ 1024,
@@ -90,7 +91,6 @@ public final class FormatTest {
             C.ENCODING_PCM_24BIT,
             /* encoderDelay= */ 1001,
             /* encoderPadding= */ 1002,
-            "language",
             /* accessibilityChannel= */ Format.NO_VALUE,
             /* exoMediaCryptoType= */ null);