Fix the race between entitlement start/stop

For very quickly hotspot OFF/ON, there is race between tethering
start entitlement from Settings and stop entitlement from wifi's
intent broadcast. Entitlement would only be started when Settings
call #startTethering(). But there are two points that tethering
would stop entitlement. One is #stopTethering() from Settings.
Another one is wifi tell tethering that it is shuting down.

In this change, second point of stop entitlement is removed to avoid
this race problem. Entitlement may have chance that do not be stopped
(e.g. network reset). This is not great but the behavior is the same
as P.
TODO: Don't reply on caller to stop entitlement. Refactor the design
to call #stopProvisioningIfNeeded when IpServer leaving TETHERED state.
TODO: mWifiTetherRequested may also suffer this race condition.

Bug: 137034691
Test: -build, flash, boot
      -atest FrameworkNetTests
      -Enable hotspot with entitlement supported SIM,
       then changing the AP band.

Change-Id: I63f858356b4543b5bdda41d03e45adc944b86da3