Migrate from isNotSameAs to isNotSameInstanceAs.

The two behave identically, and isNotSameAs is being removed.

More information:
  go/issameas-lsc

Tested:
    TAP --sample for global presubmit queue
    http://test/OCL:244736857:BASE:244751659:1555988098671:1e0f72c5
PiperOrigin-RevId: 244886651

library/core/src/test/java/com/google/android/exoplayer2/text/ttml/TtmlRenderUtilTest.java

@@ -54,8 +54,8 @@ public final class TtmlRenderUtilTest {
     String[] styleIds = {"s0", "s1"};
 
     TtmlStyle resolved = TtmlRenderUtil.resolveStyle(null, styleIds, globalStyles);
-    assertThat(resolved).isNotSameAs(globalStyles.get("s0"));
-    assertThat(resolved).isNotSameAs(globalStyles.get("s1"));
+    assertThat(resolved).isNotSameInstanceAs(globalStyles.get("s0"));
+    assertThat(resolved).isNotSameInstanceAs(globalStyles.get("s1"));
     assertThat(resolved.getId()).isNull();
 
     // inherited from s0