Import classes from direct Guava dependency

[pedro-carneiro]

This change updates some Guava class imports, in order to have them reference the classes from the project's Guava dependency, instead of the repackaged version the google-http-client provides.

[codecov[bot]]

Codecov Report

Merging #47 into master will not change coverage. The diff coverage is n/a.

@@            Coverage Diff            @@
##             master      #47   +/-   ##
=========================================
  Coverage     76.54%   76.54%           
  Complexity      191      191           
=========================================
  Files            26       26           
  Lines           904      904           
  Branches         59       59           
=========================================
  Hits            692      692           
  Misses          179      179           
  Partials         33       33

Impacted Files	Coverage Δ	Complexity Δ
...com/spotify/google/cloud/pubsub/client/Pubsub.java	76.51% <ø> (ø)	51 <0> (ø)	:arrow_down:
...a/com/spotify/google/cloud/pubsub/client/Json.java	66.66% <ø> (ø)	4 <0> (ø)	:arrow_down:

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[danielnorberg]

Thanks!

[danielnorberg]

Included in 1.33.