Remove AbstractConfig class

[msridhar]

AbstractConfig has only had one subclass for a long time, and I don't see any future scenario in which we add more. Removing AbstractConfig makes adding new NullAway options easier. We push down the code from AbstractConfig into ErrorProneCLIFlagsConfig and make the fields private and final instead of protected.

[yuxincs]

Nice cleanup!

[codecov]

Codecov Report

Attention: Patch coverage is 86.76471% with 9 lines in your changes missing coverage. Please review.

Project coverage is 85.94%. Comparing base (646b7ab) to head (50c3440).

Files	Patch %	Lines
...va/com/uber/nullaway/ErrorProneCLIFlagsConfig.java	86.76%	5 Missing and 4 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #974      +/-   ##
============================================
- Coverage     85.97%   85.94%   -0.03%     
+ Complexity     2046     2045       -1     
============================================
  Files            82       81       -1     
  Lines          6760     6761       +1     
  Branches       1301     1302       +1     
============================================
- Hits           5812     5811       -1     
- Misses          536      537       +1     
- Partials        412      413       +1     

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