Fix Shell's injection of MainCoroutineDispatcher

Since the return type of provideMainDispatcher() and
provideBackgroundDispatcher() where changed to return a
MainCoroutineDispatcher, the injection points of these also need be
updated or dagger can't provide them.
Since the CoroutineScopes weren't used yet there's no build errors
until another class actually tries to inject either the dispatchers
or the scopes.

Bug: None
Test: inject either scope and check build succeeds
Flag: EXEMPT bugfix
Change-Id: I7685259683134b6498ac7095dba06797a3e9c44a