Include getMainUser() calls on dumpsys user.

That method is deprecated but still widely used, so it will be useful
to track its usage.

This is a temporary / simpler approach - and only available on
debuggable builds - the final solution will also report to statsd
(and include unit tests).

Bug: 414326600
Bug: 418215389
Test: adb shell dumpsys user --deprecated-calls
Test: adb shell dumpsys user | grep -A 4 "called getMainUser"

Flag: EXEMPT debugging changes

Change-Id: I15625b5671b3ef23f3de621fd98aaa8562f84666