Add configurable download location via Storage Access Framework

[sahilpatel2521]

Task/Issue URL: #8832

Description

Add configurable download location via Storage Access Framework.

• Let users pick a custom download folder from Settings while preserving default Downloads behavior for existing users. Closes [Feature request] Include a setting that lets the user choose where they want to point their Downloads folder to #8832.
• Declare DownloadLocationSettingsActivity in AndroidManifest.xml.
• Reuse downloadsLocationSettingTitle for activity label to fix missing resource error and inherit translations.
• Replace nonexistent OneLineSwitchItem with OneLineListItem using showSwitch in layout.
• Pass correct buildTreeDocumentUri to folder picker initial URI.
• Add fallback download location snackbar messaging in context/webview fragments.

Steps to test this PR

Download Settings Navigation

• Open settings and verify that the Downloads option is visible under "Other Settings" (it was previously missing).
• Tap Downloads and verify that the DownloadLocationSettingsActivity launches successfully without throwing an ActivityNotFoundException.

Custom Folder Selection (Storage Access Framework)

• On the Downloads screen, tap Change Folder.
• Verify that the system folder picker opens and starts inside the primary Download folder.
• Create or select a custom folder (e.g. Sahil) and authorize access.
• Verify that the selected folder name is displayed under the Selected Folder section.

Downloading Files

• Default Location: Reset or keep the setting on "Default". Download a file from the browser and verify it saves to the default public Download folder.
• Custom Location: Set a custom folder. Download a file and verify it is successfully saved to the custom folder.
• Ask Every Time: Toggle Ask Every Time on. Attempt to download a file and verify the system asks for destination folder each time.

Fallback Verification (Folder Revoked/Unavailable)

• Select a custom folder, then revoke its storage permission or delete the folder.
• Download a file and verify that the fallback snackbar appears saying: "Selected download folder is unavailable. Downloads will be saved to the default Downloads folder."
• Verify that the file is successfully downloaded and saved in the default Download folder.

Automated Tests

• Run the tests:

  ./gradlew :downloads:downloads-impl:testDebugUnitTest
  

UI Changes

Before	After

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Note

Medium Risk
Changes the core browser download write path and persisted storage permissions; fallback behavior limits data loss risk but regressions could affect where files land or whether open/share works.

Overview
Adds configurable download folders via Storage Access Framework: a new Downloads entry under Other Settings opens DownloadLocationSettingsActivity, where users can pick a custom tree URI (persisted in DataStore) or keep the default public Downloads directory.

The download stack is refactored around DownloadDestination and DownloadDestinationResolver: AndroidFileDownloader resolves the destination before enqueueing work, serializes it through FileDownloadWorker, and DownloadFileWriter writes either to disk or SAF content:// URIs. If the chosen folder is no longer accessible, downloads fall back to default and UI surfaces ShowDownloadLocationFallbackMessage snackbars (browser, Duck Chat, subscriptions).

DownloadFileAccessor and path-based openFileAtPath / shareFileAtPath replace direct File usage in the downloads list and notifications so finished items work for both filesystem paths and content URIs. A Maestro releaseTest flow covers navigating to download location settings.

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