Speed up WriteKeepSet when generate_conditional_proguard_rules is off.

The results of CollectLocations() are only used if the --proguard-conditional-keep-rules option is set, but the function is always called when the --proguard option is set.

Since this function can be slow on particularly large resource sets, avoid calling it unless --proguard-conditional-keep-rules is set.

Change-Id: If259d41cdab16abad8ca62f30d8beb5ebf33575a
Bug: 144236322
Test: aapt2_tests