OTA only adds Main if isMainUserPermanentAdmin

On OTA to Android U, we bestow upon one user the MainUser title. Not all
devices need to have a MainUser though, depnding on their configuration.
Nonetheless, we were previously always adding MainUser on OTA.

For HSUM devices that do NOT have config_isMainUserPermanentAdmin, we should
not be setting a MainUser on OTA, so we fix this here.

On non-HSUM, we continue to make user 0 the MainUser on OTA regardless
of configuration; non-HSUM user 0 being Main is just an invariant.

Moreover, we update the emulation logic to also take into account
config_isMainUserPermanentAdmin and the invariance.
The updated logic of emulation is:

non-HSUM -> emulate HSUM
Rule: in HSUM, we never have Sys be Main. We set a new main iff the config says to.
Scenarios:
* if main=Sys (usual phone case): Remove Main from system; if isMainUserPermanentAdmin(), switch/set a new Main.
* if no main (severly aberrant case): if isMainUserPermanentAdmin(), set a new Main.
* if main=other (aberrant case): leave as is

HSUM -> emulate non-HSUM
Rule: in non-HSUM, we always make Sys to be Main
Scenarios:
* if main=Sys (severely aberrant case): leave as is
* if no main (usual for Auto):  always make Sys the Main
* if main=other (usual for tablet): always switch Main to system

Test: manual OTA from T to U
Test: manual emulate different modes
Bug: 272295293
Change-Id: Id915203fe07b65cdd0e7e08df92fe93454be4f6f