refactor: simplify `*Connection` current policy getters

[dbolduc]

Part of the work for #9168

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This change is 

[google-cloud-cpp-bot]

Google Cloud Build Logs For commit: 19b09e49b875c92b9f4c915746518052222dbfa3

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[codecov[bot]]

Codecov Report

Merging #9673 (19b09e4) into main (9276075) will increase coverage by 0.00%. The diff coverage is 99.09%.

@@           Coverage Diff           @@
##             main    #9673   +/-   ##
=======================================
  Coverage   94.33%   94.34%           
=======================================
  Files        1494     1494           
  Lines      138961   139024   +63     
=======================================
+ Hits       131089   131159   +70     
+ Misses       7872     7865    -7     

Impacted Files	Coverage Δ
...den/internal/golden_kitchen_sink_connection_impl.h	50.00% <ø> (-27.78%)	:arrow_down:
...lden/internal/golden_thing_admin_connection_impl.h	100.00% <ø> (ø)
generator/internal/connection_impl_generator.cc	48.48% <66.66%> (ø)
...en/internal/golden_kitchen_sink_connection_impl.cc	78.21% <100.00%> (+2.93%)	:arrow_up:
...den/internal/golden_thing_admin_connection_impl.cc	93.35% <100.00%> (+0.44%)	:arrow_up:
...olden/tests/golden_kitchen_sink_connection_test.cc	98.77% <100.00%> (+0.09%)	:arrow_up:
...golden/tests/golden_thing_admin_connection_test.cc	99.60% <100.00%> (+0.02%)	:arrow_up:
...bigtable/examples/bigtable_hello_instance_admin.cc	81.00% <0.00%> (-2.00%)	:arrow_down:
...cloud/pubsub/internal/subscription_session_test.cc	97.98% <0.00%> (-0.34%)	:arrow_down:
google/cloud/pubsub/samples/samples.cc	90.77% <0.00%> (+0.07%)	:arrow_up:
... and 4 more

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[dbolduc]

I am closing this PR, because I won't pick up the work any time soon. Maybe I will revisit it later.