Support fastboot variable 'variant'. Bug: 79480454 Bug: 78793464 Test: fastboot getvar variant Change-Id: Iefef82c147d8405318c793ad0a73f00674bbb63d Merged-In: Iefef82c147d8405318c793ad0a73f00674bbb63d (cherry picked from commit 4785287a0043ddea2f784433d8a4fc8a168efc79)