Gmail sync was importing messages from folders users had explicitly
disabled because the folder filtering logic lived in the parent service
but the Gmail driver wasn't aware of the import policy when building its
API queries.
Moved messageFolderImportPolicy down to the driver level so Gmail can
- Build proper -label: exclusions during initial sync
- Added `buildGmailLabelSearchName` as our syntax for nested folders was
wrong
- Filter out messages from disabled folders during incremental sync via
history API
