fix: support AWS_DEFAULT_REGION env var

[lsirac]

AWS_DEFAULT_REGION env var should be checked after AWS_REGION. See b/182491887.

[codecov]

Codecov Report

Merging #599 (5b1864e) into master (bf3507c) will increase coverage by 0.09%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master     #599      +/-   ##
============================================
+ Coverage     83.49%   83.59%   +0.09%     
- Complexity      600      602       +2     
============================================
  Files            42       42              
  Lines          2709     2712       +3     
  Branches        288      289       +1     
============================================
+ Hits           2262     2267       +5     
+ Misses          304      303       -1     
+ Partials        143      142       -1     

Impacted Files	Coverage Δ	Complexity Δ
...tp/java/com/google/auth/oauth2/AwsCredentials.java	95.32% <100.00%> (+2.05%)	17.00 <0.00> (+2.00)

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