Update the comment on LOW_POWER_MODE_TRIGGER_LEVEL in PowerUI

I originally envisioned to keep this logic and the one in the
battery service (which decides when to send a LOW_BATTERY broadcast)
in sync.

However, because this PowerUI logic decides when to show the "battery low"
notification, and showing the notification when the battery level
is still 50% doesn't really make sense even when the battery saver
starts at 50%, so let's only cap the value in PowerUI.

So if LOW_POWER_MODE_TRIGGER_LEVEL is 50,
- Battery saver kicks in at battery level 50%
- LOW_BATTERY broadcast is also sent at 50%

- But we don't show the "battery low" notification until 15% still.

(I'll talk with the PM and we might tweak the details.)

Bug: 71707409
Test: build & boot
Change-Id: I162dc722a6f634e5844ed81385a8463797634a02