Handle null activity in DMLPM calcualtions

When a task is being restored (e.g after reboot) it already contains its
relevant activities and thus no activity may be passed to the launch
param modifiers. In the event the DesktopModeLaunchParamsModifier
receives a null activity during its bound calculations, use the task top
most actiivty top resolve bounds. If the task does not contain a top
activity, simply default to the desired initial bounds.

Flag: EXEMPT bug fix
Fixes: 418046700
Test: atest WmTests:DesktopModeLaunchParamsModifierTests
Change-Id: Icb847bbc2c2b385810bd39f71ae2ddea7e75a9d0