[MEP] Use switchToSub API with pending intent

It is based on the framework refactor. ag/16494924  Framework
changed the API, so here modifies it.

Bug: 205504646
Test: WIP manual
Change-Id: I800b26c0eed8243a5f1c5ae8b7c42d1f9cfdb205
Merged-In: I800b26c0eed8243a5f1c5ae8b7c42d1f9cfdb205