Add persistent storage module

[CDRussell]

Task/Issue URL: https://app.asana.com/1/137249556945/project/488551667048375/task/1211766481496469?focus=true

Description

Implements the persistent-storage module to be a reusable, key-based abstraction over Google Play Block Store.

Changes since the previous placeholder approach in a lower branch

• New PersistentStorageKey base class — modules define their own keys with a key string and shouldBackupToCloud flag
• Operations are now store/retrieve/clear with ByteArray values and Result return types
• Availability model simplified to Available(isEndToEndEncryptionSupported) and Unavailable (removed BuildTypeUnsupported and split encrypted/unencrypted variants)

Implementation changes:

• Extracted BlockStoreWrapper to isolate Block Store API calls behind a testable interface
• DummyPersistentStorage (F-Droid) returns Unavailable / Result.failure with logcat for debugging

Observer changes:

• SyncAutoRecoveryCapabilityObserver now early-returns on F-Droid builds via AppBuildConfig.flavor check (instead of relying on BuildTypeUnsupported availability)

Steps to test this PR

Internal/Play build on emulator with Play Services

1. Install: ./gradlew installInternalDebug
2. Launch app, wait ~15s for privacy config download
3. Filter logcat: message~:"Block Store|Sync-Recovery"
4. Verify logs show:
   • Sync-Recovery: capability detection enabled (write=true, read=true)
   • Block Store: available, E2E encryption supported = true
   • Sync-Recovery: persistent storage available with E2E encryption
   • Sync-Recovery: attempting to read from persistent storage...
   • Sync-Recovery: read success
   • Sync-Recovery: attempting to write to persistent storage...
   • Block Store: stored com.duckduckgo.sync.derisk.test (N bytes)
   • Sync-Recovery: write success

Second launch (verifies cross-session persistence)
5. Force-stop and relaunch the app
6. Verify read returns previous session's value: read success (value=N chars)

Internal/Play build on device without Play Services (e.g., AOSP emulator)
7. Install: ./gradlew installInternalDebug on an emulator without GMS
8. Launch app, wait ~15s for privacy config download
9. Filter logcat: message~:"Block Store|Sync-Recovery"
10. Verify logs show:
- Sync-Recovery: capability detection enabled (write=true, read=true)
- Block Store: Play Services not available
- Sync-Recovery: persistent storage unavailable
11. Verify no read/write attempts — observer exits after unavailable pixel

F-Droid build (validates no-crash on builds without Play Services)
12. Apply patch to enable fdroidDebug variant (comment out variantFilter in app/build.gradle)
13. Install: ./gradlew installFdroidDebug
14. Launch app — verify no crash
15. Filter logcat: message~:"Block Store|Sync-Recovery" — verify no logs (observer early-returns before touching Block Store)
16. Re-enable variant filter after testing

diff --git a/app/build.gradle b/app/build.gradle
index abc1234..def5678 100644
--- a/app/build.gradle
+++ b/app/build.gradle
@@ -179,7 +179,7 @@
-    variantFilter { variant ->
-        def names = variant.flavors*.name
-        if (names.contains("fdroid") && variant.buildType.name == "debug") {
-            // We don't need fdroidDebug build type
-            setIgnore(true)
-        }
-    }
+    // variantFilter { variant ->
+    //     def names = variant.flavors*.name
+    //     if (names.contains("fdroid") && variant.buildType.name == "debug") {
+    //         // We don't need fdroidDebug build type
+    //         setIgnore(true)
+    //     }
+    // }

[CDRussell]

• Add persistent storage module #7877 👈 (View in Graphite)
• develop

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