[sanitizer][NFC] Remove InternalScopedString::size()

[sanitizer][NFC] Remove InternalScopedString::size()

size() is inconsistent with length().
In most size() use cases we can replace InternalScopedString with
InternalMmapVector.

Remove non-constant data() to avoid direct manipulations of internal
buffer. append() should be enought to modify InternalScopedString.