Fix integer sanitizer in FastMixerDumpState.

Prevent FastMixerDumpState::dump(int) from throwing a runtime error on
integer sanitized builds.

 runtime error: unsigned integer overflow: 3912 - 36680 cannot be
 represented in type 'unsigned int'

The overflow looks intentional, so this refactors the code to use
the builtin overflow function.

Bug: 30969751
Test: Compiled and device boots.
Change-Id: Ic84a1bd08839fe1af7f3b5318cd8d6ccee777443
Merged-In: Ic84a1bd08839fe1af7f3b5318cd8d6ccee777443