libsnapshot:snapuserd: Add 2MB scratch space in COW file

Add 2MB scratch space in the COW file. This is a preparation
patch for read-ahead patch. This just add the buffer
space right after the header. Bump up the version number
in the header in order to distiguish between older and newer
COW formats.

No operation is done on this buffer with this patch-set.

Scratch space option is disabled by default.

Bug: 183863613
Test: 1: Create Full OTA with the new COW format.
2: Incremental OTA with older COW format.
3: vts_libsnapshot_test

Signed-off-by: Akilesh Kailash <akailash@google.com>
Change-Id: I42a535a48ec22adb893dfe6f86a4f51650e1f88a