when storage is low, immediately trigger CleanCacheWorker run

refs #1869

Script to test this in an emulator with `adb root` in `adb shell`:
```bash
cd /data/data/org.fdroid.fdroid.debug/files
rm -f fake.apk; touch fake.apk;  chown u0_a159.u0_a159 fake.apk ; dd if=/dev/zero of=fake.apk bs=1M count=635; touch -d 2020-02-02 fake.apk ; df -h; ls -lh
```

      <

app/src/main/java/org/fdroid/fdroid/receiver/DeviceStorageReceiver.java

@@ -18,9 +18,8 @@ public class DeviceStorageReceiver extends BroadcastReceiver {
         if (Intent.ACTION_DEVICE_STORAGE_LOW.equals(action)) {
             int percentageFree = Utils.getPercent(Utils.getImageCacheDirAvailableMemory(context),
                     Utils.getImageCacheDirTotalMemory(context));
-            if (percentageFree > 2) {
-                CleanCacheWorker.schedule(context);
-            } else {
+            CleanCacheWorker.force(context);
+            if (percentageFree <= 2) {
                 DeleteCacheService.deleteAll(context);
             }
         }

app/src/main/java/org/fdroid/fdroid/work/CleanCacheWorker.java

@@ -10,6 +10,8 @@ import androidx.annotation.NonNull;
 import androidx.annotation.RequiresApi;
 import androidx.work.Constraints;
 import androidx.work.ExistingPeriodicWorkPolicy;
+import androidx.work.ExistingWorkPolicy;
+import androidx.work.OneTimeWorkRequest;
 import androidx.work.PeriodicWorkRequest;
 import androidx.work.WorkManager;
 import androidx.work.Worker;
@@ -42,7 +44,7 @@ public class CleanCacheWorker extends Worker {
     }
 
     /**
-     * Schedule or cancel a work request to update the app index, according to the
+     * Schedule or cancel a work request to clean up caches, according to the
      * current preferences. Should be called a) at boot, b) if the preference
      * is changed, or c) on startup, in case we get upgraded.
      */
@@ -68,6 +70,18 @@ public class CleanCacheWorker extends Worker {
         Utils.debugLog(TAG, "Scheduled periodic work for cleaning the cache.");
     }
 
+    /**
+     * Force a cache cleanup.  Since {@link #deleteOldInstallerFiles(Context)}
+     * only deletes files older than an hour, any ongoing APK install processes
+     * should not have their APKs are deleted out from under them.
+     */
+    public static void force(@NonNull final Context context) {
+        OneTimeWorkRequest cleanCache = new OneTimeWorkRequest.Builder(CleanCacheWorker.class).build();
+        WorkManager workManager = WorkManager.getInstance(context);
+        workManager.enqueueUniqueWork(TAG + ".force", ExistingWorkPolicy.KEEP, cleanCache);
+        Utils.debugLog(TAG, "Enqueued forced run for cleaning the cache.");
+    }
+
     @NonNull
     @Override
     public Result doWork() {
@@ -95,7 +109,10 @@ public class CleanCacheWorker extends Worker {
 
     /**
      * {@link org.fdroid.fdroid.installer.Installer} instances copy the APK into
-     * a safe place before installing.  It doesn't clean up them reliably yet.
+     * a safe place before installing.  This only deletes files older than an
+     * hour to avoid deleting APKs while they are still being installed.  This
+     * also avoids deleting the nearby swap repo files since that might be
+     * actively in use.
      */
     private void deleteOldInstallerFiles() {
         File filesDir = getApplicationContext().getFilesDir();