GN: guard tools (except fiddle) by skia_enable_tools.

Our tools use third-party libraries pretty freely, some of which may not
be available in other GN environments (e.g. Fuchsia).  Most can also not
function when Skia is built as a shared library.

fiddle stands alone as the exception to both those points: it depends on
only Skia, and works fine with both a shared or static library.

So guard everything but fiddle with this flag skia_enable_tools, disabled
when we're building for Fuchsia or when we're build a shared library.

This CL has a couple of little tweaks to Fiddle to keep it working:
  - divorce it from :tool_utils, instead just building SkForceLinking.cpp itself;
  - fix up a buggy rebase_path() that was accidentally working when we depended
    on :tool_utils;
  - drop test_only: it now only requires production-code dependencies.

The SkImageEncoder Create* methods need to be SK_API if we want SkForceLinking
to work across .so's.  Without this, SkForceLinking needs to be part of Skia; it
can't be part of the application using Skia.

The rest is mostly just a re-indent under if (skia_enable_tools),
courtesy of gn format.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2273823003

No public API changes.
TBR=reed@google.com

Review-Url: https://codereview.chromium.org/2273823003

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