Fix BatterySaverPolicy night mode flag parsing.

BatterySaverPolicy incorrectly negated the enable_night_mode flag,
so when the flag was set to true, night mode was disabled, and vice
versa. This didn't affect the default value, just when the key was
set via string constants.

Bug: 127356043
Test: atest com.android.server.power.batterysaver.BatterySaverPolicyTest
Change-Id: I45d0fc30ea0348eefc706cf67c8820d0cbb5f3a1