lrcinstance: remove custom setter/getter for currentAccountId

Using a custom getter setter cause the setting of the value to
require manual changed signal emission and in addition to being
error prone, this prevents the changed signal from being emitted
upon initial selection. Some excess workaround code may be removed
in following commits.

This patch also refactors the banned contact signaling as part of
an accountadapter connection cleanup.

Change-Id: I73fb751001e53e086adc7a6a8d706671c2878a77