ThumbnailsCacheManager - AsyncTask to Executor

[alexknop]

Problem:
-Android.os.AsyncTask is deprecated
-imageViewReference could not be null in some parts
-if/else could be optimized
-getAvatarDimension is unused

Solution
-Leverage Executor for InitDiskCacheTask
-Removed non-null check
-Use requireNonNullElseGet
-Remove unused function

[github-actions]

Codacy

Lint

Type	master	PR
Warnings	51	51
Errors	11	11

SpotBugs

Category	Base	New
Bad practice	65	65
Correctness	58	60
Dodgy code	292	279
Experimental	1	1
Internationalization	7	7
Malicious code vulnerability	1	1
Multithreaded correctness	7	7
Performance	51	48
Security	18	18
Total	500	486

[github-actions]

APK file: https://www.kaminsky.me/nc-dev/android-artifacts/14897.apk



To test this change/fix you can simply download above APK file and install and test it in parallel to your existing Nextcloud app.

[github-actions]

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