Keep close_idle_connections optimization for all devices

Internal compatibility testing didn't reveal any issues related to it.

Rationale to not make it specific to low-ram:
1) Consistent behavior from app perspective. close_idle_connections
   is a behavioural change and may affect apps relying on undocumented
   behavior of connection pooling. Developers can detect problems
   without testing on low-ram device
2) Closing idle connections is especially important in WAL mode.
   Disabling this feature would become an issue if more apps are
   switching to WAL and we would need to re-enable this optimization.

Test: n/a
Bug: 63398887
Change-Id: I1b431f44daa2d337aad7f12f8e1409ae3143ded0