Estimate COW image size during OTA generation

Estimate COW image size and put the estimation in OTA metadata. Then VAB
could use this to allocate disk space and prompt the user if more space
required.

Test: create an OTA package
Change-Id: Iaedafcf39af2d1a4d9cae9cd1a642a3cd3a4815c
diff --git a/BUILD.gn b/BUILD.gn
index 30ba977..6e282f5 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -344,6 +344,7 @@
     "payload_generator/block_mapping.cc",
     "payload_generator/boot_img_filesystem_stub.cc",
     "payload_generator/bzip.cc",
+    "payload_generator/cow_size_estimator_stub.cc",
     "payload_generator/deflate_utils.cc",
     "payload_generator/delta_diff_generator.cc",
     "payload_generator/delta_diff_utils.cc",