SkPDF: Fix encoding of unichr outside of basic plane

In ToUnicode table, write unicode codepoints as one or two UTF16BE
values, rather than a single hex, as the standard requires.

Factor out uint16 -> big-endian hex code.

SkUtils is now a namespace.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2120533002

Review-Url: https://codereview.chromium.org/2120533002
3 files changed