Add methods to decompose a RetryAlgorithm.

[igorbernstein2]

This is extracted from https://github.com/googleapis/gax-java/pull/613/files#r229890531.
It is necessary for future integration with OpenCensus to add annotation for completed attempts

[codecov-io]

Codecov Report

Merging #632 into master will decrease coverage by 0.03%.
The diff coverage is 0%.

@@             Coverage Diff             @@
##             master    #632      +/-   ##
===========================================
- Coverage     74.93%   74.9%   -0.04%     
  Complexity      935     935              
===========================================
  Files           177     177              
  Lines          4102    4104       +2     
  Branches        328     328              
===========================================
  Hits           3074    3074              
- Misses          875     877       +2     
  Partials        153     153

Impacted Files	Coverage Δ	Complexity Δ
...va/com/google/api/gax/retrying/RetryAlgorithm.java	75% <0%> (-10.72%)	7 <0> (ø)

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[vam-google]

LGTM