Addresses failures with the following, in some files:
-Wshorten-64-to-32
-Wimplicit-int-conversion
-Wsign-compare
-Wsign-conversion
-Wtautological-unsigned-zero-compare

(This specific CL focuses on .cc files in strings/, except /internal/.)

Bug: chromium:1292951
PiperOrigin-RevId: 465285043
Change-Id: I37e9d1b4c4e9aa655b720da1467927af2aba995e