libutils: Limit integer sanitize to target only

This matches what the Android.mk defined, and should temporarily fix
builds that were broken with:

system/core/libutils/Unicode.cpp:225:12: runtime error: unsigned integer
overflow: 0 - 1 cannot be represented in type 'size_t' (aka 'unsigned
long')

Change-Id: I0363b42fc2d62dfd2d05649c9aa9ef0be573e20a