Support fastboot variable 'variant'.

Bug: 79480454
Bug: 78793464
Test: fastboot getvar variant
Change-Id: Iefef82c147d8405318c793ad0a73f00674bbb63d
Merged-In: Iefef82c147d8405318c793ad0a73f00674bbb63d
(cherry picked from commit 4785287a0043ddea2f784433d8a4fc8a168efc79)
4 files changed