Allow BTM_BLE_CONN_INT_MIN_LIMIT override by sysprop

For mainline (and VF) builds we want to allow customization for the
BTM_BLE_CONN_INT_MIN_LIMIT through a sysprop. Since it's too late to
have it as an API for T at this point, or it's sepolicy, this will be
properly added in aosp. For T the sepolicy can be added in system_ext
(e.g. for TV it can be added in
device/google/atv/sepolicy/system_ext/private).

Bug: 237014662
Test: local build with sysprop
Ignore-AOSP-First: TM only fix
Change-Id: I8b2dafeaebf4fa8fae434bff0c937db9c14f3f9b