adbd: allowing adb root when the device is unlocked

As there is no security guarantee when the device is unlocked,
allowing adb root gives us more rooms to debug a USER build images.

Also, this makes it possible to run VTS on a USER build GSI, with
setting ro.debuggable=1 and unlocking the device.

This basically re-lands a reverted change:
https://android-review.googlesource.com/c/platform/system/core/+/437815

Which isn't needed after we moved /sbin/adbd to /system/bin/adbd in
USERDEBUG GSI. But it's still needed for USER build GSI.

Bug: 126493225
Test: unlock a USER build device, check 'adb root' can work
Change-Id: I93f12c8a3fe65c96c947e4602795eadfe591c521