Revert "Replace mPendingRemove with hierarchy deferred removal"

This reverts commit bb2efa02710451cee3cc566e67315ed8ba53f430.

Reason for revert: This breaks the dialogs exit animation. See
b/163976519#comment24.

Bug: 163976519
Test: CtsActivityManagerDeviceTestCases
Change-Id: Id15ca74ebd4a9f3e58c99d13a3c8e8b275b96814
3 files changed