Update low battery notification mechanism

 - Update warning and severe trigger percentage
 - Update low battery notification trigger condition to fixed value instead of remaining time
 - Update low battery notification text
 - Update start saver confirmation notification text
 - Use severe low battery dialog if available
 Screenshot: https://screenshot.googleplex.com/iZt43YU6fHQPNpy.png

Bug: 207470943
Bug: 209932572
Test: atest SystemUITests
Change-Id: I851e635e748a5315f9780a1344c2068cda14d95e