Replace isSubscriptionEnabled with isActiveSubId.

As we don't allow pSIM modem disablment, we should simply use isActiveSubId
to decide whether a subscription's preferences is changable. This will
avoid potential modem bugs that reports wrong modem status that result
in Setting's UX error.

Bug: 135222940
Test: manual and robo
Change-Id: I7cccf2fdab7c89d26dac4daad51cd5d6f3a90eba