fix(common): backoff policy clones initial state

[dbolduc]

When calling .clone() on an exponential backoff policy, copy the initial state instead of the current state. This may result in a change of behavior, but we see it as a bug that ought to be fixed.

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This change is 

[google-cloud-cpp-bot]

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[codecov]

Codecov Report

Merging #7696 (8b9ec64) into main (edbe664) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #7696   +/-   ##
=======================================
  Coverage   95.29%   95.29%           
=======================================
  Files        1254     1254           
  Lines      113579   113589   +10     
=======================================
+ Hits       108233   108249   +16     
+ Misses       5346     5340    -6     

Impacted Files	Coverage Δ
google/cloud/bigtable/rpc_backoff_policy.cc	100.00% <100.00%> (ø)
google/cloud/bigtable/rpc_backoff_policy.h	100.00% <100.00%> (ø)
google/cloud/bigtable/rpc_backoff_policy_test.cc	100.00% <100.00%> (ø)
google/cloud/internal/backoff_policy.cc	100.00% <100.00%> (ø)
google/cloud/internal/backoff_policy.h	100.00% <100.00%> (+8.33%)	⬆️
google/cloud/internal/backoff_policy_test.cc	100.00% <100.00%> (ø)
google/cloud/bigtable/internal/common_client.h	94.02% <0.00%> (-5.98%)	⬇️
google/cloud/pubsub/samples/samples.cc	92.10% <0.00%> (+0.07%)	⬆️
...cloud/pubsub/internal/subscription_session_test.cc	98.00% <0.00%> (+0.24%)	⬆️
...ud/spanner/integration_tests/client_stress_test.cc	86.18% <0.00%> (+0.65%)	⬆️
... and 3 more

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[coryan]

Consider updating CHANGELOG.md as part of this PR, it will be harder to remember what we wanted to say when we release this.

[devbww]

Possible follow-up note: It seems to me that copy construction/assignment for these "clone-able" classes is an attractive nuisance (they don't support equality after all), and that we'd be well served trying to delete that capability.

[google-cloud-cpp-bot]

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