[aapt2] Fix infinite loop in proguard::CollectLocations

std::set only works correctly when the < comparator is a strict weak
ordering, while UsageLocation::operator< was actually implementing !=.

Bug: 134190468
Change-Id: Icb9407e9c8451f9fcb4eb9b2cea310e3bcaf159e
Tested: aapt2_tests, and b/134190468#comment1