Change back default allocator to pooled.

Motivation:

While porting some changes from 4.0 to 4.1 and master branch I changed the default allocator from pooled to unpooled by mistake. This should be reverted. The guilty commit is 4a3ef90.

Thanks to @blucas for spotting this.

Modifications:

Revert changes related to allocator.

Result:

Use the correct default allocator again.

buffer/src/main/java/io/netty/buffer/ByteBufUtil.java

@@ -53,7 +53,9 @@ public final class ByteBufUtil {
             HEXDUMP_TABLE[(i << 1) + 1] = DIGITS[i       & 0x0F];
         }
 
-        String allocType = SystemPropertyUtil.get("io.netty.allocator.type", "unpooled").toLowerCase(Locale.US).trim();
+        String allocType = SystemPropertyUtil.get(
+                "io.netty.allocator.type", PlatformDependent.isAndroid() ? "unpooled" : "pooled");
+        allocType = allocType.toLowerCase(Locale.US).trim();
 
         ByteBufAllocator alloc;
         if ("unpooled".equals(allocType)) {