Fix resolveImeUserIdLocked bug on non-concurrent multi-user mode

`I03f6f4b08470afc36eb8d7381b14556804d6b2c2` improved IMMS#resolveImeUserIdLocked, which is used by showSoftInput and hideSoftInput to resolve the user id from the display owner when the caller id was SYSTEM_USER. However, this also introduced a regression for non-concurrent multi-user mode. The regression is that, in non-concurrent multi-user mode, it should return mCurrentImeUserId instead of callingProcessUserId.

Bug: 362665493
Bug: 358105425
Flag: EXEMPT bug fix
Test: presubmit
Change-Id: I79cba7f9e3aefc87ef28254656871b7bac9e3d35