use FlashingPlan sparse_limit for testing

Have get_sparse_limit() return FlashingPlan's sparse limit if set. Used
for testing to compare task lists

Test: fastboot_test
Change-Id: I7ac1eb4dc83a53b3d7aa59aa668a780b4b550664
diff --git a/fastboot/task.h b/fastboot/task.h
index 500655d..858f43a 100644
--- a/fastboot/task.h
+++ b/fastboot/task.h
@@ -82,7 +82,7 @@
 class FlashSuperLayoutTask : public Task {
   public:
     FlashSuperLayoutTask(const std::string& super_name, std::unique_ptr<SuperFlashHelper> helper,
-                         SparsePtr sparse_layout, uint64_t super_size);
+                         SparsePtr sparse_layout, uint64_t super_size, const FlashingPlan* fp);
     static std::unique_ptr<FlashSuperLayoutTask> Initialize(const FlashingPlan* fp,
                                                             std::vector<ImageEntry>& os_images);
     static std::unique_ptr<FlashSuperLayoutTask> InitializeFromTasks(
@@ -96,6 +96,7 @@
     std::unique_ptr<SuperFlashHelper> helper_;
     SparsePtr sparse_layout_;
     uint64_t super_size_;
+    const FlashingPlan* fp_;
 };
 
 class UpdateSuperTask : public Task {