[PERF] Import - Avoid copying local files to cache

Came across this when looking into cache usage for the visual editor. I had 700MB in my cache (my phone seems buggy and shows this as "data" usage rather than "cache" on the app screen).

It was removed when I cleared the cache.

File was in `/data/data/com.ichi2.anki/cache`

This an easy win in terms of import performance and removing "out of space" errors. It'll also reduce the likelihood of #5880 as we won't leave remnant files in the cache.

###### Reproduction Steps

1. Import
2. Select File from Internal Storage

###### Expected Result

3. File is not copied to cache when local

###### Actual Result

3. File is copied to cache

###### Debug info
AnkiDroid Version = 2.10alpha70

Android Version = 9

ACRA UUID = d84098a2-8ccd-4e01-b2cf-51d99e7b02c5

###### Research
- [x] I have read the [support page](https://ankidroid.org/docs/help.html) and am reporting a bug or enhancement request specific to AnkiDroid
- [x] I have checked the [manual](https://ankidroid.org/docs/manual.html) and the [FAQ](https://github.com/ankidroid/Anki-Android/wiki/FAQ) and could not find a solution to my issue
- [x] I have searched for similar existing issues here and on the user forum