fix(cells): secure file sharing [WPB-26687] PR 6

[MohamadJaara]

https://wearezeta.atlassian.net/browse/WPB-26687

Goal

Keep the narrowed FileProvider surface introduced by this stack while restoring Cells file open and share flows.

Issue

1. Download a file through Cells.
2. Open it in an external app or share it.
3. Cells passes the file from the app-specific external-files directory directly to FileProvider.
4. The narrowed provider paths reject that location, so URI creation fails.

Changes

• Move the secure sharing helpers into core:ui-common so both the app and Cells use the same policy.
• Link or copy only the selected Cells file into the dedicated file-provider-shares cache directory before creating its URI.
• Keep pre-Android 15 Wire targets excluded from the chooser.
• Enable trusted share identity on Android 15 and newer.
• Add regression coverage for Cells open/share flows and shared-cache path confinement.

Security

The provider configuration is not widened. External apps receive a temporary read grant for a URI under the dedicated shared-cache root only, and untrusted display names cannot escape that directory.

Stack

1. #5033
2. #5034
3. #5035
4. #5036
5. #5037
6. This PR

Depends on #5037.