Don't use std::stoi()

We use it for parsing out the userIds in paths passed in by user code,
and invalid inputs could cause std::stoi() to throw an exception and
crash the process.

Bug: 172629984
Test: cd /storage/emulated/1000000000000
Change-Id: Iaafffb1684ef0d719d4927458dfad45989d76971
1 file changed