Update instructions for Android and our sample app

[msridhar]

Fixes #891. It's hard to add a regression test for this, but I manually tested that with the new configuration, NullAway errors are detected in the sample app. After this lands, I'd like to update the readme further to refer to this particular commit, so users can see the changes we needed to make.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (091ac38) 86.99% compared to head (96b696b) 86.99%.

Additional details and impacted files

@@            Coverage Diff            @@
##             master     #900   +/-   ##
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  Coverage     86.99%   86.99%           
  Complexity     1958     1958           
=========================================
  Files            77       77           
  Lines          6319     6319           
  Branches       1223     1223           
=========================================
  Hits           5497     5497           
  Misses          418      418           
  Partials        404      404           

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[yuxincs]

Documentation updates LGTM.

Not saying that we should do this, but perhaps if this breaks more often we can consider adding an integration test CI job that sets this up and use regex to check that the output contains the expected errors 😃

[msridhar]

Not saying that we should do this, but perhaps if this breaks more often we can consider adding an integration test CI job that sets this up and use regex to check that the output contains the expected errors 😃

Yeah I'm hoping it doesn't come to that 🙂 But if needed, that would be a good way to test for regressions.