Adjust low battery notification

 - Make low battery notification as ongoing notification and remove the
   delete intent since ongoing notification can not been remove
 - Remove unused attr
 Screenshot: https://screenshot.googleplex.com/gj5xYDik5Ct2WQb.png

Bug: 207470943
Test: atest SystemUITests
Change-Id: I9cfdc8deb455bb5f6707c08ffff3ff3405e99f2b