Fixing EthFilter serialisation issue

[kamilargent]

What does this PR do?

Fixes the issue with EthFilter serialisation that is happening on Android that's using the latest Android Gradle Plugin and proguard for code minification.

Where should the reviewer start?

EthFilter.java

Why is it needed?

getThis is treated as this property that during serialisation fails with direct self-reference leading to cycle

The exact stacktrace:

com.fasterxml.jackson.databind.exc.InvalidDefinitionException: Direct self-reference leading to cycle (through reference chain: org.web3j.protocol.core.Request["params"]->java.util.Arrays$ArrayList[0]->org.web3j.protocol.core.methods.request.EthFilter["this"])
	at com.fasterxml.jackson.databind.SerializerProvider.reportBadDefinition(SerializerProvider.java:1300)
	at com.fasterxml.jackson.databind.ser.BeanPropertyWriter._handleSelfReference(BeanPropertyWriter.java:944)
	at com.fasterxml.jackson.databind.ser.BeanPropertyWriter.serializeAsField(BeanPropertyWriter.java:722)
	at com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeFields(BeanSerializerBase.java:774)
	at com.fasterxml.jackson.databind.ser.BeanSerializer.serialize(BeanSerializer.java:178)
	at com.fasterxml.jackson.databind.ser.impl.IndexedListSerializer.serializeContents(IndexedListSerializer.java:119)
	at com.fasterxml.jackson.databind.ser.impl.IndexedListSerializer.serialize(IndexedListSerializer.java:79)
	at com.fasterxml.jackson.databind.ser.impl.IndexedListSerializer.serialize(IndexedListSerializer.java:18)
	at com.fasterxml.jackson.databind.ser.BeanPropertyWriter.serializeAsField(BeanPropertyWriter.java:728)
	at com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeFields(BeanSerializerBase.java:774)
	at com.fasterxml.jackson.databind.ser.BeanSerializer.serialize(BeanSerializer.java:178)
	at com.fasterxml.jackson.databind.ser.DefaultSerializerProvider._serialize(DefaultSerializerProvider.java:480)
	at com.fasterxml.jackson.databind.ser.DefaultSerializerProvider.serializeValue(DefaultSerializerProvider.java:319)
	at com.fasterxml.jackson.databind.ObjectMapper._writeValueAndClose(ObjectMapper.java:4568)
	at com.fasterxml.jackson.databind.ObjectMapper.writeValueAsString(ObjectMapper.java:3821)
	at org.web3j.protocol.Service.send(Service.java:46)
	at org.web3j.protocol.core.Request.send(Request.java:87)
	at org.web3j.protocol.core.Request$$ExternalSyntheticLambda0.call(R8$$SyntheticClass:0)
	at org.web3j.protocol.core.Remote<truncated: 6151 chars>
       at com.fasterxml.jackson.databind.SerializerProvider.reportBadDefinition(SerializerProvider.java:1300)
       at com.fasterxml.jackson.databind.ser.BeanPropertyWriter._handleSelfReference(BeanPropertyWriter.java:944)
       at com.fasterxml.jackson.databind.ser.BeanPropertyWriter.serializeAsField(BeanPropertyWriter.java:722)
       at com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeFields(BeanSerializerBase.java:774)
       at com.fasterxml.jackson.databind.ser.BeanSerializer.serialize(BeanSerializer.java:178)
       at com.fasterxml.jackson.databind.ser.impl.IndexedListSerializer.serializeContents(IndexedListSerializer.java:119)
       at com.fasterxml.jackson.databind.ser.impl.IndexedListSerializer.serialize(IndexedListSerializer.java:79)
       at com.fasterxml.jackson.databind.ser.impl.IndexedListSerializer.serialize(IndexedListSerializer.java:18)
       at com.fasterxml.jackson.databind.ser.BeanPropertyWriter.serializeAsField(BeanPropertyWriter.java:728)
       at com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeFields(BeanSerializerBase.java:774)
       at com.fasterxml.jackson.databind.ser.BeanSerializer.serialize(BeanSerializer.java:178)
       at com.fasterxml.jackson.databind.ser.DefaultSerializerProvider._serialize(DefaultSerializerProvider.java:480)
       at com.fasterxml.jackson.databind.ser.DefaultSerializerProvider.serializeValue(DefaultSerializerProvider.java:319)
       at com.fasterxml.jackson.databind.ObjectMapper._writeValueAndClose(ObjectMapper.java:4568)
       at com.fasterxml.jackson.databind.ObjectMapper.writeValueAsString(ObjectMapper.java:3821)
       at org.web3j.protocol.Service.send(Service.java:46)
       at org.web3j.protocol.core.Request.send(Request.java:87)