OMS: extract verifyIdmap from createIdmap

For clarity, split IIdmap2::createIdmap into two separate functions:

  - IIdmap2::verifyIdmap [check if an existing idmap file is OK to use]
  - IIdmap2::createIdmap [unconditionally (re)create an idmap file]

Teach the IdmapManager to call verifyIdmap and to proceed with
createIdmap only if actually needed.

Test: atest OverlayDeviceTests OverlayHostTests
Change-Id: I9f6f1192011fcb094adffeca1eb3f709520bbd24
diff --git a/cmds/idmap2/idmap2d/Idmap2Service.h b/cmds/idmap2/idmap2d/Idmap2Service.h
index 2b32042..4e5abc9 100644
--- a/cmds/idmap2/idmap2d/Idmap2Service.h
+++ b/cmds/idmap2/idmap2d/Idmap2Service.h
@@ -39,6 +39,9 @@
   binder::Status removeIdmap(const std::string& overlay_apk_path, int32_t user_id,
                              bool* _aidl_return);
 
+  binder::Status verifyIdmap(const std::string& overlay_apk_path, int32_t user_id,
+                             bool* _aidl_return);
+
   binder::Status createIdmap(const std::string& target_apk_path,
                              const std::string& overlay_apk_path, int32_t user_id,
                              std::unique_ptr<std::string>* _aidl_return);