New J2K: implement mutability inference in post-processing

#KT-24293 fixed
#KT-19603 fixed

nj2k/testData/copyPaste/AddImports.expected.kt

@@ -1,5 +1,5 @@
 import java.io.File
 
-internal fun foo(file: File?): List<String> {
-    return emptyList()
+internal fun foo(file: File?): List<String?>? {
+    return emptyList<String>()
 }

nj2k/testData/copyPaste/AddImportsWithExplicitImports.expected.kt

@@ -1,7 +1,7 @@
 import java.io.File
 
 class C {
-    private fun memberFun(file: File?) {}
+    private fun memberFun(file: File) {}
 
     companion object {
         @JvmStatic

nj2k/testData/copyPaste/MethodWithNoAnnotation.expected.kt

@@ -1,5 +1,5 @@
 package to
 
-fun foo(): String {
+fun foo(): String? {
     return ""
 }

nj2k/testData/copyPaste/MethodWithOnlyOneAnnotation.expected.kt

@@ -1,6 +1,6 @@
 package to
 
 @SomeAnnotation
-fun foo(): String {
+fun foo(): String? {
     return ""
 }

nj2k/testData/copyPaste/SeveralMethodsSample.expected.kt

@@ -23,12 +23,12 @@ class A {
         return JetRefactoringUtil.formatPsiMethod(element as PsiMethod, true, false)
     }
 
-    protected fun getDimensionServiceKey(): String {
+    protected fun getDimensionServiceKey(): String? {
         return "#org.jetbrains.kotlin.idea.refactoring.safeDelete.KotlinOverridingDialog"
     }
 
-    fun getSelected(): ArrayList<UsageInfo> {
-        val result: ArrayList<UsageInfo> = ArrayList<UsageInfo?>()
+    fun getSelected(): ArrayList<UsageInfo>? {
+        val result: ArrayList<UsageInfo> = ArrayList<UsageInfo>()
         for (i in 0 until myChecked.length) {
             if (myChecked.get(i)) {
                 result.add(myOverridingMethods.get(i))