Do not reject valid connection parameter updates

As a side-effect to increasing BTM_BLE_CONN_INT_MIN, valid connection
parameter updates can be rejected if the requested minimum is < 11.25ms,
but the requested maximum is >= 11.25ms.

This patch introduces a new parameter "BTM_BLE_CONN_INT_LIMIT" and
resets BTM_BLE_CONN_INT_MIN to the HCI spec defined minimum.

Bug: 22245224
Change-Id: I6a2e4d18488b63570f7d0070413d00f16a7fdba1