Fix doFadingAnimationLocked in sanitized builds.

The loop as constructed causes i to overflow twice when i = 0 on integer
overflow sanitized builds.

 runtime error: unsigned integer overflow: 0 - 1 cannot be represented
 in type 'size_t' (aka 'unsigned long')

 runtime error: unsigned integer overflow: 18446744073709551615 + 1
 cannot be represented in type 'size_t' (aka 'unsigned long')

This refactors the loop to avoid the overflows.

Bug: 30969751
Test: Compiles, device boots, enabled pointer location overlay.
Change-Id: I844bb3b84b1f536c50d06fb489fcc22590d4aa98