Removed the POLICY_ALLOW_BACKGROUND_BATTERY_SAVE constant.

That constant was created back in 2014 in the intial Battery Saver
implementation, but it's not used anymore (since the power saving
whitelist is fetched from DeviceIdleController).

Test: m -j32 FrameworksCoreTests &&  adb install -r -g
${ANDROID_PRODUCT_OUT}/data/app/FrameworksCoreTests/FrameworksCoreTests.apk
&& adb shell am instrument -e class
"android.net.NetworkPolicyManagerTest" -w
"com.android.frameworks.coretests/android.support.test.runner.AndroidJUnitRunner"

BUG: 28791717

Change-Id: Ic2c0bcb9736ce8b40e04fd644953e68241d06a69