[Android] Add .cxx directories to app template .gitignore - #161069

Thanks!

One nit below. I also did a quick search for existing gitignore files this should perhaps be added to: $ git ls-files | grep android/.gitignore Other than the ones already in this PR, that finds:

engine/src/flutter/shell/platform/android/.gitignore
engine/src/flutter/testing/scenario_app/android/.gitignore
packages/integration_test/android/.gitignore

I think the second one is a true match that should get this change. The third I'm not sure about. The first is a false positive.

Oh and this search for relevant templates:

$ git ls-files packages/flutter_tools/ | grep android.*gitignore
packages/flutter_tools/templates/app/android.tmpl/.gitignore
packages/flutter_tools/templates/plugin/android.tmpl/.gitignore
packages/flutter_tools/templates/plugin_shared/android.tmpl/.gitignore

scenario_app actually doesn't use the flutter tool, so I don't think it should be impacted here by the change to force ndk download (it's days are numbered regardless - it was a way to test behavior in something like a full flutter app, and essentially replicated whatever framework/tool functionality was needed for the test).

packages/integration_test/android/.gitignore - I added it here as well. I don't know that it will matter (I haven't seen the cxx directories being created by my change for plugin directories), but our plugin templates include it in the gitignore so I think it is fine to add it anyways.

The app/ one is included here, and plugin_shared/ already has this line. I don't understand the difference between plugin/ and plugin_shared/ — should plugin/ get this too?

Also added to the plugin/ template. My understanding is that generally when both a foo and foo_shared directory exist, that the output of the template is a function of both of the directories. But I also know that the files that don't end in tmpl get copied as is, so I'm not sure how that is possible here (perhaps one of the .gitignore files just wins?) Either way, I've checked the output of the template manually and verified that the .gitignore contains the .cxx/ line we want (both before and after adding to the plugin/ file).