Don't try to install system apps without APK

during restore process. These can usually not be manually installed anyway and just clutter the list making it harder for the user to find their important apps and potential failures there.

app/src/main/java/com/stevesoltys/seedvault/restore/install/ApkRestore.kt

@@ -63,6 +63,9 @@ internal class ApkRestore(
             // The @pm@ package needs to be included in [backup], but can't be installed like an app
             if (packageName == MAGIC_PACKAGE_MANAGER) return@mapNotNull null
             // we don't filter out apps without APK, so the user can manually install them
+            // exception is system apps without APK, as those can usually not be installed manually
+            if (metadata.system && !metadata.hasApk()) return@mapNotNull null
+            // apps that made it here get a state class for tracking
             ApkInstallResult(
                 packageName = packageName,
                 state = if (isAllowedToInstallApks) QUEUED else FAILED,

app/src/test/java/com/stevesoltys/seedvault/restore/install/ApkRestoreTest.kt

@@ -427,6 +427,32 @@ internal class ApkRestoreTest : TransportTest() {
         }
     }
 
+    @Test
+    fun `system app without APK get filtered out`() = runBlocking {
+        // only backed up package is a system app without an APK
+        packageMetadataMap[packageName] = PackageMetadata(
+            time = 23L,
+            system = true,
+            isLaunchableSystemApp = Random.nextBoolean(),
+        ).also { assertFalse(it.hasApk()) }
+
+        every { installRestriction.isAllowedToInstallApks() } returns true
+        every { storagePlugin.providerPackageName } returns storageProviderPackageName
+
+        apkRestore.installResult.test {
+            awaitItem() // initial empty state
+            apkRestore.restore(backup)
+
+            awaitItem().also { finishedItem ->
+                println(finishedItem.installResults.values.toList())
+                // the only package provided should have been filtered, leaving 0 packages.
+                assertEquals(0, finishedItem.total)
+                assertTrue(finishedItem.isFinished)
+            }
+            ensureAllEventsConsumed()
+        }
+    }
+
     @Test
     fun `no apks get installed when blocked by policy`() = runBlocking {
         every { installRestriction.isAllowedToInstallApks() } returns false