fix: keep keyboard appearance as same brightness as system theme

[hyj1204]

Fix #239

The keyboard appearance will use the same brightness(light or dark) as the current system theme.

[codecov]

Codecov Report

Merging #264 (2a7cd9a) into main (ed75bf6) will increase coverage by 0.20%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #264      +/-   ##
==========================================
+ Coverage   62.41%   62.61%   +0.20%     
==========================================
  Files         246      246              
  Lines       10847    10857      +10     
==========================================
+ Hits         6770     6798      +28     
+ Misses       4077     4059      -18     

Impacted Files	Coverage Δ
...tor_component/service/ime/delta_input_service.dart	0.00% <ø> (-0.81%)	⬇️
...component/service/ime/non_delta_input_service.dart	64.00% <ø> (+4.74%)	⬆️
...itor_component/service/ime/text_input_service.dart	100.00% <100.00%> (ø)
...tor_component/service/keyboard_service_widget.dart	88.03% <100.00%> (+0.20%)	⬆️

... and 16 files with indirect coverage changes