reconcile and rename fuchsia variables used in GN build rules

[richkadel]

flutter/buildroot build/fuchsia/sdk.gni has a declare_args section that defines a set of "global" variables reused by many other .gni and BUILD.gn files, such as fuchsia_sdk_root.

Some of these variables are ambiguous (fuchsia_sdk_root vs. fuchsia_sdk_path, for instance), and some variable names may misrepresent the state of the build environment.

For example, using_fuchsia_gn_sdk = false. As part of flutter/engine#26880, I'm currently updating DEPS to import the Fuchsia GN SDK (instead of the Fuchsia SDK), but it doesn't appear to be possible (or at least, practical) to set using_fuchsia_gn_sdk = true. I suspect that the variable's value may not actually correspond to whether flutter is building with the Fuchsia GN SDK, or with the Fuchsia SDK, so if the variable still has value, it would help to update the variable name, and expand on the associated comment in sdk.gni to better reflect its purpose.

Perhaps this should be done as part of #67373 (if it's easier to make all of these changes at once), or separately if not.