Make "Want to start battery saver?" dialog better.

- Don't line-break in "Battery" (only in English)
- Add "Learn more" link.

Bug: 78261259

Test: Manual test with:
- adb shell dumpsys battery unplug
- adb shell settings delete secure low_power_warning_acknowledged
- Enable battery saver
- Make sure the link is clickable if a link is set.
- Make sure there's no "learn more" link if the link is not set in strings.xml

Cherry picked I83364f628dd596a4d50bf2aca4db7cbfe7cf4909 and manually resolved conflict.

Change-Id: Icf6d4e62d8cb3556561f992afc45aaea868bce0e