WIP: java: move CredentialsProvider up to ClientSettings

[pongad]

redo of #1251.

WIP pending more tests, but all generated code's
unit tests run OK against updated gax.

Adding Neo since a similar change broke his code earlier.

[codecov-io]

Codecov Report

Merging #1319 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##             master    #1319      +/-   ##
============================================
+ Coverage     86.47%   86.47%   +<.01%     
  Complexity     3838     3838              
============================================
  Files           378      378              
  Lines         15332    15336       +4     
  Branches       1696     1696              
============================================
+ Hits          13258    13262       +4     
  Misses         1623     1623              
  Partials        451      451

Impacted Files	Coverage Δ	Complexity Δ
...nsformer/java/JavaGapicSurfaceTestTransformer.java	100% <100%> (ø)	27 <0> (ø)	⬇️
.../transformer/java/JavaGapicSurfaceTransformer.java	98.13% <100%> (+0.01%)	43 <0> (ø)	⬇️

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[garrettjonesgoogle]

LGTM - this will work as an intermediate state until my refactoring is done.

[garrettjonesgoogle]

This seems good to go, is there anything left to do?

[pongad]

I forgot to update the baseline. Will do that and merge.