Initialize stable bounds during onDragStart

The display appearance is occurring after the task
appearance. Because of this, we cannot initialize the stable bounds in
the task positioners' constructors.

Test: atest CtsWindowManagerDeviceTestCases -- --abi x86_64
Bug: 289101765
Change-Id: Ia994ba2eb81a558a53de17839c8959b830bff970
2 files changed