Fix a race condition when testing trade-in mode.

It's possible for trade-in mode to race with normal userdebug adbd when
using "tradeinmode testing start". We previously tried to fix this using
property triggers, but it's cleaner to simply set the tradeinmode
persist property before rebooting.

Bug: 381182849
Flag: EXEMPT bugfix
Test: adb shell tradeinmode testing start
Change-Id: Ibb07c379f0947721a11c6fc521bf3c784b54016c