Revert "Revert "Decide the current user after PHASE_THIRD_PARTY boot phase""

This reverts commit 9cd24f63.

An issue preventing starting user 0 will be addressed as a separate CL.

Bug: 124460424
Test: Test system bootup (for 1st boot and subsequent boot) with next patch
Change-Id: I319bdcefaf1b7a1b7484f27cc4374778ee9318aa