Increase max system_server hwbinder threadpool to 5.

system_server hosts HIDL services, but also many different
callback interfaces. Subtle deadlocks can occur if there's
only one HwBinder thread to handle incoming transactions.
Increasing this to 5 threads seems reasonable - we haven't
really seen any performance issues due to this.

The main danger with this change is that not all callback
interfaces may be written in a thread-safe way, but some
quick testing revealed no issues.

Bug: 37531523
Bug: 37509431
Test: builds, Maps/YouTube/Camera/Music/Netflix
Change-Id: I52bff2f2263625c1a33f60b8b27c307c1d31000c