De-duplicate entries written with AAPT2 convert

There was no check for whether we had already written a specific file path to the APK when using the convert command.

If the resources table points 2 resource IDs at the same file on disk, the convert command would write the file twice, creating two entries.

This holds a set of file paths already written and ignores duplicates.

Bug: 123271593

Test: Ran convert on linked bug's weird.apk
Test: aapt2_tests case for duplicate entries

Change-Id: Ia22515bf8e8297624aaadbf6a9e47159026c63e5