Take margin insets into account when setting position

Previously we computed insets for task menu position
whenever a Task was told that orientation had changed.
This missed a path for the TaskMenuView's onScrollChanged()
listener, which also calls set position and wasn't taking
the insets into account.

Moved the inset into setPosition() directly so it's caught
on all code paths.

Also noticed 2 bugs where
* We were calling updateChildTaskOrientation() twice, it
already happens in the call to updateOrientationHandler()
* We were directly modifying insets from the activity's
drag layer instead of copying those insets to another rect
and then modifying (this is no longer an issue since we
are not touching those insets at all anymore)

Bug: 192400086
Test: Rotated w/ task menu open for fake and real
landscape. Nothing seems broken.
Note real landscape hides the menu whereas fake one
shows it (which was behavior before this change)

Change-Id: I613dac9519220f49285655ef11a1f72e4a6d31bd