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)
diff --git a/fastboot/constants.h b/fastboot/constants.h
index ad2e1a1..c11d9de 100644
--- a/fastboot/constants.h
+++ b/fastboot/constants.h
@@ -61,3 +61,4 @@
 #define FB_VAR_IS_USERSPACE "is-userspace"
 #define FB_VAR_HW_REVISION "hw-revision"
 #define FB_VAR_VARIANT "variant"
+#define FB_VAR_OFF_MODE_CHARGE_STATE "off-mode-charge"