Add support to read fastboot variable 'off-mode-charge'

Bug: 78793464
Bug: 79480454

Test: fastboot getvar off-mode-charge
Change-Id: I4c40847be292e8e2e420340f81bb624b247bc11b
Merged-In: I4c40847be292e8e2e420340f81bb624b247bc11b
(cherry picked from commit b05d278696cbb25e14085362651999204aa51b40)
4 files changed