[Application] Fix launcher idle callback return value and EP message loop type

1. init_extension_process_channel should return FALSE, otherwise it will be
called endlessly.
2. Thread running EP logic should have a TYPE_DEFAULT message loop type. As
on Linux system, the launcher's main thread already has a Glib message pump(TYPE_UI).
Merge pull request #1832 from seanlong/launcher_fix

[Application] Fix launcher idle callback return value and EP message loop type