Update .gitignore (#28789)

Summary:
When you profile your heap and memory allocations with Memory Profiler, files with *.hprof extension are created in /android folder that has big sizes (up to 600 MB for each). These files may be needed to add to gitignore.

## Changelog

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[Android] [Added] - Add *.hprof files to gitignore
Pull Request resolved: https://github.com/facebook/react-native/pull/28789

Differential Revision: D21432927

Pulled By: hramos

fbshipit-source-id: a20f12645de5ca0874c9130094e2f97fe16b2203