Check for preexisting flag before enabling the gadget

onRegistration callback gets triggered as soon has the
serviceNotification object is registered with registerForNotifications.
The preexisting flag in the callback is set if the gadget hal service dies
and restarts. Use this to differentiate between the onRegistration call
for the registerForNotifications vs actual gadget hal service restart.
Do not reset usb gadget if the onRegistration callback is called as a
result of registerForNotifications. This will overcome the occasional
reset of the usb gadget hence the connection during phone bootup.

Bug: 118678530
Test: Verified that gadget reset does not happen
Change-Id: I3b94e55289bb3a53eb9e083177fd4d54bec5f4da