Add support for reading modules.load.charger when booting into charger mode

When booting up, Android can boot into one of three modes: normal,
recovery, and charger mode. The set of modules that should be loaded
during first stage init in each mode can differ, which is why init
reads the list of modules to load from modules.load.recovery when
booting into recovery, and modules.load otherwise.

This means that init will read the list of modules to load during first
stage init from modules.load even when booting into charger mode. This
is not ideal, as it causes modules that need to be loaded during
first stage init only when booting into charger mode to also be loaded
during first stage init of normal boot, which can degrade boot time.

Thus, add support for reading modules.load.charger, which contains the
list of modules that need to be loaded during first stage init when
booting into charger mode.

Bug: 266752750
Change-Id: Ib9178bdfe5a6aac57b86b6d453b03625e95d5b48
Signed-off-by: Isaac J. Manjarres <isaacmanjarres@google.com>