[CrOS Hotspot] Checking thethering readiness before enabling hotspot

Skipping cached value and checking tethering readiness before enabling
hotspot.

BUG=b/326140624

Change-Id: I21f31738f95b067b04b5838798615d49176a8a69
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5394739


Reviewed-by: Jason Zhang <jiajunz@google.com>
Commit-Queue: Nikhil Nayunigari <nikhilcn@google.com>
Cr-Commit-Position: refs/heads/main@{#1280243}