feat(android): select and safely update Alpha or Nightly builds

[veryCrunchy]

Goal

Let direct-APK installations choose a release channel and update safely inside Nextcloud Native without assuming Play Store or F-Droid ownership.

Scope

• offer Alpha and Nightly for direct APK installations
• represent Beta and Stable as future-compatible channels without offering unavailable builds
• persist and migrate the selected channel safely
• hide the selector for store-owned installations
• bind update metadata and install actions to the selected channel
• reject downgrades, stale cross-channel results, noncanonical metadata, checksum mismatches, package mismatches, and signer mismatches
• recognize Android platform package-source classifications, including OEM local/download installers

Acceptance criteria

• changing channels invalidates results from the previous channel
• installation revalidates the current selected channel
• only a strictly newer Android version code can install
• direct APK and store-owned installations are distinguished conservatively
• focused common and Android tests cover migration, source classification, channel binding, downgrade protection, and metadata validation
• no signing keys or private installation data enter the repository

Parent epic: #16