Cap the low battery threshold at config_lowBatteryWarningLevel (=15)

Currently it doesn't matter because LOW_POWER_MODE_TRIGGER_LEVEL can be 15%
at most and config_lowBatteryWarningLevel is 15, but because
we'll allow bigger LOW_POWER_MODE_TRIGGER_LEVEL, we don't want to do
the low battery warning at a battery level too high.

Bug: 71707409
Test: manual test with "dumpsys battery unplug" and "dumpsys battery set level XX"
Change-Id: Ibd3646decef243bbed21c148134ab1d3930bea14