[refactoring] Wrap calls to Types.subst and Types.memberType

[msridhar]

This is in preparation to attempt to fix #1091. Just a refactoring, no behavior changes.

[codecov]

Codecov Report

Attention: Patch coverage is 93.33333% with 1 line in your changes missing coverage. Please review.

Project coverage is 88.13%. Comparing base (17df87f) to head (7bde4cd).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
.../uber/nullaway/generics/TypeSubstitutionUtils.java	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##             master    #1115   +/-   ##
=========================================
  Coverage     88.13%   88.13%           
- Complexity     2229     2231    +2     
=========================================
  Files            84       85    +1     
  Lines          7223     7232    +9     
  Branches       1437     1437           
=========================================
+ Hits           6366     6374    +8     
- Misses          430      431    +1     
  Partials        427      427           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.