Enable the "not-callable" pylint check

Enable the "not-callable" pylint check

This check catches attempts to call variables that pylint believes are
not functions.  A trivial example would be:

    # Trivial example caught by this check:
    foo = dict()
    print foo('bar')  # <- oops, meant foo['bar']

This change enables the "not-callable" pylint check, after disabling a
few cases where the alert triggers but the usage was intended (defining
decorators).

Change-Id: I09ad929902509018fe7183a15b784601c36b6196
Related-Bug: #1356224