incfs: Create incomplete folder

incfs 2 also creates a .incomplete folder, so we need to set correct
permissions on that folder too

Bug: 169084168
Test: With updated kernel, incfs_test works and adb install
	--incremental works
Change-Id: Id7b9d017973777e37628664530dfed18c55c269f

services/incremental/IncrementalService.cpp

@@ -481,6 +481,9 @@ StorageId IncrementalService::createStorage(std::string_view mountPoint,
         if (!mkdirOrLog(path::join(backing, ".index"), 0777)) {
             return kInvalidStorageId;
         }
+        if (!mkdirOrLog(path::join(backing, ".incomplete"), 0777)) {
+            return kInvalidStorageId;
+        }
         auto status = mVold->mountIncFs(backing, mountTarget, 0, &controlParcel);
         if (!status.isOk()) {
             LOG(ERROR) << "Vold::mountIncFs() failed: " << status.toString8();