Restore TW_BACKUP_AVG_IMG_RATE handling

Preserving the average BPS across partitions appears to have been
wiped out by the adb backup patch set. Restore it.

Change-Id: Id05c12ff61260b642ee7acd979132a2cba026d87
diff --git a/partitionmanager.cpp b/partitionmanager.cpp
index a772ea4..f94b6aa 100644
--- a/partitionmanager.cpp
+++ b/partitionmanager.cpp
@@ -872,6 +872,7 @@
 
 	int prev_img_bps = 0, use_compression = 0;
 	unsigned long long prev_file_bps = 0;
+	DataManager::GetValue(TW_BACKUP_AVG_IMG_RATE, prev_img_bps);
 	img_bps += (prev_img_bps * 4);
 	img_bps /= 5;