Set mCurrentUserId before UC.startUserInternal returns.

UserController.startUserInternal method was broken into two parts after
ag/31995061, executing the first half on the caller's thread,
and the last half on the UserController.mHandler thread, in order
to make sure onBeforeUserSwitching event is called for the
UserSwitchObservers on UserController.mHandler thread, and to be synced
with the other events of UserSwitchObservers. Otherwise, it was causing
a race condition. More info can be found on the commit message of the
CL with the Change-Id I3b9de74fc74fc586656ea962b4efafc2442cf5d1

While that CL was fixing the race condition, it was changing the behavior
of the UserController.startUserInternal method. Before the CL, the
method was setting mCurrentUserId in case of a foreground user start,
before returning. But now, the mCurrentUserId is set on a different
thread, which can lead to some unexpected results for the already existing
parts of the platform.

This CL makes the behavior of the UserController.startUserInternal method
more similar to how it was before ag/31995061, without losing the fix.

Bug: 378175388
Test: atest UserControllerTest
Flag: EXEMPT bugfix
Change-Id: I236ad92d4f6e8c936ff8045d3a8c6d351645c4cd