Update SkBits2Float and SkFloat2Bits to use uint32_t.

The most recent version of clang now enforces that %x must be
passed an unsigned value. SkFloat2Bits is most commonly used for
printing, so it now returns a uint32_t. For parity, its sibling
API now uses uint32_t as well. In practice, almost every call
site was already using unsigned ints, or was using bitwise math
where the signs did not matter.

Change-Id: I5c73910aa7522f749b9f4b417c06310e40529fff
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/842119
Commit-Queue: Brian Osman <brianosman@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
4 files changed