Add composite string for battery summary in settings

We needed a string with the percentage in the front. This CL
adds a composite string to create it out of an already exisitng
string rather than trying to make another power remaining
variant.

Test: robotests is sister CL
Bug: 77874406
Change-Id: I31edcc06f3a504c6400c506f961fb35b6afc6abd
(cherry picked from commit b9683d49)