Remove InferredJARModelsHandler

[msridhar]

Now, all logic for parsing loading astubx files is consolidated into the LibraryModelsHandler.ExternalStubxLibraryModels class and is uniform for astubx files generated via JarInfer or from source code, which will ease future bug fixes and changes.

We copied over the logic from InferredJARModelsHandler for loading Android SDK models if present. We also update the logic for ExternalStubxLibraryModels.nonNullParameters() and ExternalStubxLibraryModels.nullableReturns() to pull in all the relevant info from the parsed astubx file.

Fixes #1072

[codecov]

Codecov Report

Attention: Patch coverage is 88.33333% with 7 lines in your changes missing coverage. Please review.

Project coverage is 88.09%. Comparing base (9ff664a) to head (9109f86).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
...m/uber/nullaway/handlers/LibraryModelsHandler.java	87.93%	3 Missing and 4 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #1079      +/-   ##
============================================
+ Coverage     88.01%   88.09%   +0.07%     
+ Complexity     2228     2204      -24     
============================================
  Files            85       84       -1     
  Lines          7212     7175      -37     
  Branches       1428     1423       -5     
============================================
- Hits           6348     6321      -27     
+ Misses          436      431       -5     
+ Partials        428      423       -5     

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