ibusManager: Fix the panel after porting to Promises

ibusManager: Fix the panel after porting to Promises

Commit 764527c8c9b7659901eb6296a6859ae2b0eabdb8 not only ports this file
to Promises but also changes the behavior of _initPanelService method.
Instead of always calling _updateReadiness when _panelService is ready,
it only calls it when get_global_engine_async succeeds.

The only callers of _updateReadiness are _initEngines and
_initPanelService. Assume that _initEngines completes first. Its
_updateReadiness call keeps _ready as false and it is expected for
_initPanelService to change it to true. However, since
get_global_engine_async fails because there is no active engine,
_initPanelService never calls _updateReadiness. Therefore, all setEngine
calls do nothing because _ready is false, and the input method panel
never shows. Users are unable to use any input method even if they can
see that ibus-daemon is already running.

Fix the issue by changing it back to the old behavior.

https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1235