Remove unnecessary calling IdmapExist

After OverlayManagerServiceImpl calls IdmapDaemon.createIdmap that
makes sure the expected idmap file existed, it still calls
IdmapDaemon.idmapExists in calculateNewState function to check
whether the idmap file exists or not.

Once the idmap file is verified by IdmapDaemon.verifyIdmap, we
can skip calling IdmapDaemon.idmapExists to check because
IdmapDaemon.verifyIdmap did it.

Test: run perfetto.ui and follow the commands on perfetto.
    b/174206591#comment14

Bug: 174206591
Change-Id: Ib7a346688331724eb972bd1fa82f44d565a47a0d