Fix nfcsnoop_cb on overflow sanitized builds.

On builds with integer overflow sanitizer enabled, a runtime error
occurs due to an overflow when calculating a time delta:

 runtime error: unsigned integer overflow: 1513015414325284 -
 1513015414449720 cannot be represented in type 'unsigned long'

This overflow looks benign, so this refactors the code to use the
built-in overflow function so the runtime error no longer occurs.

Bug: 30969751
Test: Compiles, device boots.
Change-Id: I84e2f033c22f94ec3675336a53e50f7f794cb7a6
1 file changed