Fix unable finish booting when display added before boot

We started home activity before system ready because overlay
display was added. It accidentally updated the AM state to booted
without actually finishing booting procedure. The booting procedure
was unable to complete afterward.

Do not start home activity before system booting.

Bug: 118796241
Test: atest ActivityStackSupervisorTests

Change-Id: I4e4a38b742529fc7ba6163c1a05137c68faf83e7