Add nanoTime method to Clock to support overriding System.nanoTime()

#minor-release

PiperOrigin-RevId: 545237925
(cherry picked from commit 527073acdfa4a1f1e43bc3abb3ef7a54e0bb8696)

library/common/src/main/java/com/google/android/exoplayer2/util/Clock.java

@@ -51,6 +51,9 @@ public interface Clock {
    */
   long uptimeMillis();
 
+  /** See {@link java.lang.System#nanoTime()} */
+  long nanoTime();
+
   /**
    * Creates a {@link HandlerWrapper} using a specified looper and a specified callback for handling
    * messages.

library/common/src/main/java/com/google/android/exoplayer2/util/SystemClock.java

@@ -50,6 +50,11 @@ public class SystemClock implements Clock {
   }
 
   @Override
+  public long nanoTime() {
+    return System.nanoTime();
+  }
+
+  @Override
   public HandlerWrapper createHandler(Looper looper, @Nullable Callback callback) {
     return new SystemHandlerWrapper(new Handler(looper, callback));
   }

testutils/src/main/java/com/google/android/exoplayer2/testutil/FakeClock.java

@@ -146,6 +146,12 @@ public class FakeClock implements Clock {
   }
 
   @Override
+  public synchronized long nanoTime() {
+    // Milliseconds to nanoseconds
+    return timeSinceBootMs * 1000000L;
+  }
+
+  @Override
   public long uptimeMillis() {
     return elapsedRealtime();
   }