Respect previously resolved size if layout has no size value

Some apps such as Kindle only specify gravity, but no size in
window layout. In this case, we use the default task size for them,
but it should be better to use the previously resolved size.

Bug: 172668149
Bug: 173077136
Test: atest WmTests:TaskLaunchParamsModifierTests
Change-Id: I1f841aa85d9586785f6baf9628cb627efcf50b98