build: bump sdkMinVersion to 28 (Android 9)

[joragua]

Related Issues

App: #4880

• Add changelog files for the fixed issues in folder changelog/unreleased. More info here
• Add feature to Release Notes in ReleaseNotesViewModel.kt creating a new ReleaseNote() with String resources (if required)

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QA

[jesmrec]

Blocked till 4.8.2 is out

[DeepDiver1975]

Reviewed as maintainer. The min-SDK bump itself is clean and consistent, and CI is green — nice work. A few non-blocking observations, mostly about the "remove obsolete SDK version checks" acceptance criterion from #4880.

Looks good

• build.gradle: sdkMinVersion = 24 -> 28 is the single source of truth; all modules (owncloudApp, owncloudData, owncloudDomain, owncloudComLibrary, owncloudTestUtil) reference sdkMinVersion, so the bump propagates consistently.
• The removed guards are all genuinely redundant at minSdk 28 (API 28 = Android P): Build.VERSION_CODES.O (26), .P (28), and the < P dexopener check are now always-true / always-false, so the simplifications are correct and behaviour-preserving.
• Robolectric @Config(sdk = [..O]) -> [..P] and the DexOpener unconditional install are appropriate.
• Changelog: changelog/unreleased/4885 is present and matches TEMPLATE.md and the repo's PR-number naming convention. (Optional nit: you could also add the https://github.com/owncloud/android/issues/4880 URL line so the fragment links back to the issue.)

Leftover obsolete SDK checks the PR missed (the issue AC asks to "remove obsolete SDK version checks" — these three are now dead/redundant at minSdk 28 and were not caught):

1. owncloudApp/.../ui/preview/PreviewTextFragment.kt:248 — setRolesAccessibilityToMenuItems() still wraps its body in if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O). This is the exact same accessibility pattern you unwrapped in FileDetailsFragment, MainFileListFragment, PreviewAudioFragment, PreviewImageFragment, PreviewVideoActivity, FileDisplayActivity, FileFragment and DrawerActivity. This one appears to have been overlooked — worth unwrapping it too for consistency.
2. owncloudApp/.../utils/NotificationUtils.kt:93 — if (SDK_INT < Build.VERSION_CODES.O && timeOut \!= null) is now unreachable dead code (a workaround for API < 26 that can never run). Safe to drop the SDK_INT < O part.
3. owncloudApp/.../extensions/ContextExt.kt:27 — @RequiresApi(Build.VERSION_CODES.O) on createNotificationChannel is now redundant (minSdk 28 >= 26), especially since this PR removed the < O early-return in MainApp.createNotificationChannels().

None of these are blocking or functional regressions — they're dead-code / consistency cleanups that fall within this PR's stated scope. Recommend folding them in (at minimum #1) before merge so the "remove obsolete checks" AC is fully satisfied. The support-policy decision (dropping Android 7/8 / API 24–27) is a product call and is already covered by #4880.