Make SkFontConfigInterface_direct and sysroot-aware

CL [1] adds support for setting the fontconfig sysroot with an environment
variable.  Chromium plans on using this in tests.

The string returned when getting the FC_FILE property is relative to the
sysroot, so to get the full path, the sysroot must be prepended.

[1] https://cgit.freedesktop.org/fontconfig/commit/?id=730deada8cf609157d07b7c2bf2985672614c4c0

Bug: chromium:822737,chromium:908463
R=bungeman
Change-Id: I6d9dfbb3bdc6bdcd2aab73e757dcef46ae9229e4
Reviewed-on: https://skia-review.googlesource.com/c/127108
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>