Attempt to fix updateImeParent transaction synchronized issue

As DC#assignChildLayers is now use getSyncTransaction() to merge layers
transaction. In case updateImeParent() using getPendingTransaction() to
reparent/assignRelativeLayer for IME container that may not sync with
the layer computation, uses getSyncTransaction() in updateImeParent()
to ensure all the layer assignments can apply in the same transaction.

Bug: 204242389
Test: manual as issue steps with enabling shell transiion
Change-Id: I1c5eaa1d68a7e9e313fdbf23ed77c9cbef1f1d16