Fix wrong check in DC#needsRelativeLayeringToIme

Previous CL[1] uses isImeAttachedToApp to check if the IME should set
relative layer with the app window, but it's wrong because the method
also check if the IME parent has reparented to the app.
(This brakes RoleManagerTest failure as Bug 202246318, since the test
activity has created a child window but we accidentally set the wrong
relative layer of IME even the activity didn't request showing IME,
so the child window will end up not be on the top of screen)

As originally CL[1] is only need to check if the window is eligible to
attach IME then we don't need to set relative layer for IME,
use shouldImeAttachedToApp to fix the original issue and unblock
RoleManagerTest failure.

Also move allowAttachToApp related with magnicationSpec check from
computeImeParent into shouldImeAttachedToApp to make the attach logic
more consistent.

[1]: I2f466a2305e56e4fd2a0c509d7c8027104cb51b6

Fix: 202227155
Fix: 202246318
Bug: 201504527
Test: atest RoleManagerTest ZOrderingTests

Change-Id: Ib6233b8f9270829a7d7c6fbb3bd98fdefaf4c206