Fix sanitizer errors in bootstat.cpp.

Integer overflow sanitized builds are throwing an error on the while
loop decrement in the rfind function. This refactors the loop to prevent
decrementing the value on the final iteration.

Test: Compiled and device boots without runtime error.
Bug: 30969751
Change-Id: Ice4532cce933062b3c14adf2d9749cfdea4ad84c
Merged-In: Ice4532cce933062b3c14adf2d9749cfdea4ad84c