Extract flagged APIs in the nested classes.

Test: mmm -j frameworks/base/api/coverage/tools/:extract-flagged-apis
      extract-flagged-apis <input api.text file> <output pb file>
Bug: 311059624
Change-Id: I7db4296e4ee78268b886b83cd822d8ebe151c4a5

api/coverage/tools/ExtractFlaggedApis.kt

@@ -25,7 +25,7 @@ fun main(args: Array<String>) {
     var cb = ApiFile.parseApi(listOf(File(args[0])))
     val flagToApi = mutableMapOf<String, MutableList<String>>()
     cb.getPackages()
-        .allTopLevelClasses()
+        .allClasses()
         .filter { it.methods().size > 0 }
         .forEach {
             for (method in it.methods()) {