Eliminate unused import in Android embedding (flutter#15975)

Eliminates an unused dependency on android.os.build. The last remaining
use of this was eliminated in 0615f45
with the removal of an unused method that contained an android version
check.

shell/platform/android/io/flutter/embedding/android/FlutterFragment.java

@@ -8,7 +8,6 @@
 import android.arch.lifecycle.Lifecycle;
 import android.content.Context;
 import android.content.Intent;
-import android.os.Build;
 import android.os.Bundle;
 import android.support.annotation.NonNull;
 import android.support.annotation.Nullable;