Fuchsia: Break apart package & script gen into separate GN rules.

This is a prerequisite step toward supporting multi-package
deployments.

* Replaces all calls to fuchsia_executable_runner() with
package and script generation targets. The revised rules are
simple enough that fuchsia_executable_runner() is no longer a
major convenience.
* Delete test_runner_script() for the same reasons.
* Use sensible defaults for things like sandbox_policy to clean
up the parameters for most instances of the rules.

Bug: 	855242
Change-Id: I2f04e1baa9afaf24cfd5d979d33e9912a0475e55
Reviewed-on: https://chromium-review.googlesource.com/1110749
Reviewed-by: Dmitry Gozman <[email protected]>
Reviewed-by: Scott Graham <[email protected]>
Reviewed-by: Luke Halliwell <[email protected]>
Reviewed-by: John Budorick <[email protected]>
Commit-Queue: Kevin Marshall <[email protected]>
Cr-Commit-Position: refs/heads/master@{#569850}
9 files changed