libbinder*: ToString - pound def made consistent

- libbinder/libbinder_ndk types may be used together (for instance,
  in a system process which happens to use both), so we no longer
  disable ToString for libbinder when libbinder_ndk is included
- pound defs are undefined if they are unused

Bug: 259162245
Test: N/A

Change-Id: Ib3c6e18719d126c4bc7ab33e8d2bd3c28b219c92