[android] Unify custom map server dialog logic and UI

- Add setting icon for custom map server download option.
- Extract shared custom map server dialog helper.
- Clean up based on reviewers' feedback.

Signed-off-by: NoelClick <dev@noel.click>

libs/platform/platform.cpp

@@ -49,11 +49,6 @@ bool GetFileTypeChecked(std::string const & path, Platform::EFileType & type)
 }
 }  // namespace
 
-namespace
-{
-    std::string g_customMetaServerUrl;
-} // namespace
-
 // static
 Platform::EError Platform::ErrnoToError()
 {