Shove type into source_info

We can shove type into source info to save 8 bits in per cow operation.
We only need 4 bits inside of source_info to enumerate all the types of
Cow Operation:

static constexpr uint8_t kCowCopyOp = 1;
static constexpr uint8_t kCowReplaceOp = 2;
static constexpr uint8_t kCowZeroOp = 3;
static constexpr uint8_t kCowLabelOp = 4;
static constexpr uint8_t kCowClusterOp = 5;
static constexpr uint8_t kCowXorOp = 6;
static constexpr uint8_t kCowSequenceOp = 7;
static constexpr uint8_t kCowFooterOp = -1;

Test: critical ota paths on pixel
Change-Id: I22049db0b39a55bd8f863339f3751d3146d5c1e9