[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
diff --git a/tools/aapt2/java/ProguardRules.h b/tools/aapt2/java/ProguardRules.h
index f9656d1..b15df59 100644
--- a/tools/aapt2/java/ProguardRules.h
+++ b/tools/aapt2/java/ProguardRules.h
@@ -99,11 +99,13 @@
 //
 
 inline bool operator==(const UsageLocation& lhs, const UsageLocation& rhs) {
+  // The "source" member is ignored because we only need "name" for outputting
+  // keep rules; "source" is used for comments.
   return lhs.name == rhs.name;
 }
 
-inline int operator<(const UsageLocation& lhs, const UsageLocation& rhs) {
-  return lhs.name.compare(rhs.name);
+inline bool operator<(const UsageLocation& lhs, const UsageLocation& rhs) {
+  return lhs.name.compare(rhs.name) < 0;
 }
 
 //