Fix audioflinger in overflow sanitized builds.

The loop as constructed in Track::triggerEvents potentially leads to
two unsigned integer overflows on the i = 0 loop.

This refactors the loop to prevent the overflow.

Bug: 30969751
Test: Compiles and device boots.
Change-Id: I7ac3223ab3197f5c475a4d09c99e6f05d0ddb208
Merged-In: I7ac3223ab3197f5c475a4d09c99e6f05d0ddb208