Fix ReceiveExternalFilesActivity intent

[ZetaTom]

This change fixes a crash when the app was presented with a specially crafted intent.

Intents to share files with Nextcloud via ReceiveExternalFilesActivity.java are now checked, if they contain the appropriate actions and extras before attempting to parse them.

If an intent lacks the appropriate action or extras, an error message is displayed.

• Tests written, or not not needed

[github-actions]

APK file: https://www.kaminsky.me/nc-dev/android-artifacts/12406.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]

Codacy

Lint

Type	master	PR
Warnings	71	71
Errors	3	3

SpotBugs

Category	Base	New
Bad practice	68	68
Correctness	71	71
Dodgy code	351	351
Experimental	2	2
Internationalization	7	7
Malicious code vulnerability	2	2
Multithreaded correctness	6	6
Performance	56	56
Security	19	19
Total	582	582

[github-actions]

blue-Light-Screenshot test failed: https://www.kaminsky.me/nc-dev/android-integrationTests/12406-Screenshot-blue-Light-07-48

[github-actions]

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