[android] Runtime Style Filter enhancements (#6623)

* [android] Additional null check - prevents errors in array conversion

* [android] style conversion - delete local refs asap to avoid ref table overflow

* [android] Filter - hide concrete implementation classes to avoid confusion

platform/android/MapboxGLAndroidSDK/src/main/java/com/mapbox/mapboxsdk/style/layers/Filter.java

@@ -31,7 +31,7 @@ public class Filter {
     /**
      * Represents a {@link Filter} statement. Can be unary (eg `has()`, etc) or take any number of values.
      */
-    public static class SimpleStatement extends Statement {
+    private static class SimpleStatement extends Statement {
         private final String key;
         private final Object[] values;
 
@@ -40,7 +40,7 @@ public class Filter {
          * @param key      the property key
          * @param values   the values to operate on, if any
          */
-        public SimpleStatement(String operator, String key, Object... values) {
+        SimpleStatement(String operator, String key, Object... values) {
             super(operator);
             this.key = key;
             this.values = values;
@@ -63,14 +63,14 @@ public class Filter {
     /**
      * Represents a collection of {@link Statement}s with an operator that describes their relationship
      */
-    public static class CompoundStatement extends Statement {
+    private static class CompoundStatement extends Statement {
         private final Statement[] statements;
 
         /**
          * @param operator   the relationship operator
          * @param statements the statements to compound
          */
-        public CompoundStatement(String operator, Statement... statements) {
+        CompoundStatement(String operator, Statement... statements) {
             super(operator);
             this.statements = statements;
         }

platform/android/src/style/android_conversion.hpp

@@ -75,7 +75,9 @@ inline optional<std::string> toString(const mbgl::android::Value& value) {
 }
 
 inline optional<Value> toValue(const mbgl::android::Value& value) {
-    if (value.isBool()) {
+    if (value.isNull()) {
+        return {};
+    } else if (value.isBool()) {
         return { value.toBool() };
     } else if (value.isString()) {
         return { value.toString() };

platform/android/src/style/value.cpp

@@ -9,7 +9,11 @@ namespace android {
 
     Value::Value(jni::JNIEnv& env, jni::jobject* _value) : jenv(env), value(_value) {}
 
-    Value::~Value() {}
+    Value::~Value() {
+        if (value) {
+            jni::DeleteLocalRef(jenv, value);
+        }
+    }
 
     bool Value::isNull() const {
         return value == nullptr;