FIR: Rewrite Java type mapping

Previoisly, there were two places where mapping had happened:
- toConeKotlinTypeWithNullability
- enhancePossiblyFlexible

The first one was used for supertypes and bounds and the second one
was used for other signature parts

The main idea is to perform type mapping once to a flexible type,
and then use it as it's needed (it's lower bound, or for the further ehnancement)

Also, this commit fixes flexibility for type arguments, see the tests

compiler/testData/loadJava/compiledJava/notNull/NotNullIntArray.fir.txt

@@ -1,5 +1,5 @@
 public open class NotNullIntArray : R|kotlin/Any| {
-    @R|org/jetbrains/annotations/NotNull|() public open operator fun hi(): R|ft<kotlin/IntArray, kotlin/IntArray?>!|
+    @R|org/jetbrains/annotations/NotNull|() public open operator fun hi(): R|kotlin/IntArray|
 
     public constructor(): R|test/NotNullIntArray|