Fix integer sanitizer in audiopolicyservice.

A size_t was being converted to a ssize_t after an intended overflow
had already occurred. This makes the conversion explicit.

unsigned integer overflow: 0 - 1 cannot be represented in type 'unsigned
int'

Test: Compiled with the change and checked the sanitizer output.
Bug: 30969751
Change-Id: Ic15cc4b5d8295e14e3588ffa240830f7570dcaf3
Merged-In: Ic15cc4b5d8295e14e3588ffa240830f7570dcaf3