[application] move ThreadHost::Create out of the Application class

[Irving-cl]

This PR refactors the Application class.

In specific:

1. Decouple Application with InfraLinkSelector by moving InfraLinkSelector out of the Application class. The current dependency causes that we cannot move the creation of ThreadHost instance out of the constructor of the Application class.
2. Move Ncp::ThreadHost::Create out of the Application constructor.

This change solves the fuzzer test issue in Android: When we create an Application object, we have to initlialize the radio which will cause error in the fuzz environment. With this change, we can pass a FuzzThreadHost object to the Application.

[codecov]

Codecov Report

Attention: Patch coverage is 84.00000% with 4 lines in your changes missing coverage. Please review.

Project coverage is 46.15%. Comparing base (2b41187) to head (472a121).
Report is 859 commits behind head on main.

Files with missing lines	Patch %	Lines
src/agent/application.cpp	80.00%	0 Missing and 3 partials ⚠️
src/agent/main.cpp	88.88%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2574      +/-   ##
==========================================
- Coverage   55.77%   46.15%   -9.63%     
==========================================
  Files          87      105      +18     
  Lines        6890    12454    +5564     
  Branches        0      908     +908     
==========================================
+ Hits         3843     5748    +1905     
- Misses       3047     6394    +3347     
- Partials        0      312     +312     

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[zhanglongxia]

LGTM