Put is_skia_standalone back a bit.

Put is_skia_standalone back a bit.

We do still need to know when we're in our GN environment or in another,
like Fuchsia's or Flutter's.

The places where we replaced is_skia_standalone with defined(ndk) still
look right to me as defined(ndk).

CQ_INCLUDE_TRYBOTS=skia.primary:Build-Ubuntu-GCC-x86_64-Release-Flutter_Android

Change-Id: I5455014a8f4a350ac3a939097d042fd8cb4c32fa
Reviewed-on: https://skia-review.googlesource.com/9193
Commit-Queue: Mike Klein <mtklein@chromium.org>
Reviewed-by: Mike Klein <mtklein@chromium.org>