Extend Nexus Player mono workaround to Oreo

The platform fix was merged into O MR1, not O. Nexus Player has
not received this build.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=206734342

library/core/src/main/java/com/google/android/exoplayer2/audio/DefaultAudioSink.java

@@ -1146,7 +1146,7 @@ public final class DefaultAudioSink implements AudioSink {
 
     // Workaround for Nexus Player not reporting support for mono passthrough.
     // (See [Internal: b/34268671].)
-    if (Util.SDK_INT <= 25 && "fugu".equals(Util.DEVICE) && !isInputPcm && channelCount == 1) {
+    if (Util.SDK_INT <= 26 && "fugu".equals(Util.DEVICE) && !isInputPcm && channelCount == 1) {
       channelCount = 2;
     }