Fix sanitizer in AudioFlinger threadLoop.

The integer sanitizer was catching an unsigned integer overflow on the
calculation for throttleMs. This gets implicitly cast to a signed
integer anyways, so just perform the cast explicitly.

Bug: 30969751
Test: Compiles with and without integer sanitization.
Test: Device boots.
Change-Id: I4ef7c6f2e871ee56b81e87f3d71be8d85b222934
Merged-In: I4ef7c6f2e871ee56b81e87f3d71be8d85b222934