crosvm: plugin: use create_sandbox_minijail() for plugin

create_plugin_jail() and create_sandbox_minijail() do the same settings
except (1) `RLIMIT_NOFILE` and (2) plugin jail does not support embeded
bpf seccomp filter. But both difference won't be a problem.

(1) Given that Parallels plugin is closed source we will not be able t
ocame up with a reasonable limit, so it might need to go the way GPU
device does it and set the limit to 32K.

(2) We validate seccomp_policy_dir in jail_config is not none.

The fallback logic for the pivot root path for plugin is a bit changed,
but it should not be a problem.

* before: cfg.plugin_root > DEFAULT_PIVOT_ROOT env > "/var/empty"
* after: cfg.plugin_root > jail_config.pivot_root > DEFAULT_PIVOT_ROOT
  env > "/var/empty"

This migration also introduce minor updates to jail helpers.

* create_base_minijail() validates root path to be absolute path.
  * otherwise, chroot fails and minijail fails.
* The source for mounting proc is changed ("/proc" -> "proc").
  * "proc" looks the proper source to mount according to proc(5) man
    page.
  * "/proc" as the source was introduced by https://crrev.com/c/1717739,
    but it looks they didn't have any concrete reason for it.

BUG=b:268281422
TEST=./tools/run_tests2
TEST=tast run $DUT arc.PlayStore.vm

Change-Id: Ie014cadf3cec4a049ef783546782209e26d9902a
Reviewed-on: https://chromium-review.googlesource.com/c/crosvm/crosvm/+/4262049
Reviewed-by: Dmitry Torokhov <dtor@chromium.org>
Commit-Queue: Shin Kawamura <kawasin@google.com>