[dbus] simplify getting OT instance in dbus RCP thread object

[Irving-cl]

This PR simplifies the way to get OT instance in DbusThreadObjectRcp.

[zhanglongxia]

LGTM

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 13 lines in your changes missing coverage. Please review.

Project coverage is 43.64%. Comparing base (2b41187) to head (05ee60a).
Report is 910 commits behind head on main.

Files with missing lines	Patch %	Lines
src/dbus/server/dbus_thread_object_rcp.cpp	0.00%	13 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #2660       +/-   ##
===========================================
- Coverage   55.77%   43.64%   -12.13%     
===========================================
  Files          87      106       +19     
  Lines        6890    13254     +6364     
  Branches        0      953      +953     
===========================================
+ Hits         3843     5785     +1942     
- Misses       3047     7163     +4116     
- Partials        0      306      +306     

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