xds/resolver: minor cleanup in the config selector implementation

[easwars]

This PR injects the dependencies of configSelector at creation time, instead of passing a reference to the xdsResolver and having the former directly access fields from the latter.

RELEASE NOTES: none

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.18%. Comparing base (1059e84) to head (d0b22b4).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8609      +/-   ##
==========================================
+ Coverage   82.05%   82.18%   +0.12%     
==========================================
  Files         415      415              
  Lines       40694    40685       -9     
==========================================
+ Hits        33390    33435      +45     
+ Misses       5916     5874      -42     
+ Partials     1388     1376      -12     

Files with missing lines	Coverage Δ
internal/xds/resolver/serviceconfig.go	85.55% <100.00%> (-0.16%)	⬇️
internal/xds/resolver/xds_resolver.go	81.76% <100.00%> (-0.37%)	⬇️

... and 20 files with indirect coverage changes

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