Fix battery tip app name is empty

- Top waste app is linux process not android process. Therefore
it doesn't contains application label. Fixed by return the process
name if packages label is empty.

Fixes: 146558570
Test: follow the steps:
1. adb shell settings put global battery_tip_constants test_low_battery_tip=true
2. Unplug usb. Go to Settings -> Battery, it should show an item
"Phone used more than usual"
3. Click "phone used more than usual" item, all the items should
have title and icon.

Change-Id: I85c232bc6a048b3104c4fce0fcec2b944e8ac1b4