IME: Fix broken IME target calculation for child windows

When the IME target ends up being a starting window, we try to use the window
behind it - but fail to take into account the child windows of the window behind,
which may also be candidates for IME targets.

Also unifies the treatment of TYPE_APPLICATION_STARTING regardless of the surface
format (currently, starting windows always have alpha, so this is a no-op change,
but unblocks testing and prevents accidentially triggering the other code path should
we change the screenshot format).

Fixes: 159267713
Bug: 159290973
Test: atest 'DisplayContentTests#testComputeImeTarget_startingWindow'
Change-Id: Iedd5f7407926167f4891ce9b7e9a79e22751e668