Fix primary activity untouchable with punch hole

The root cause is that the letterbox receives touches within the whole task,
which made the primary activity untouchable.

This CL changes to make the letterbox only cover the secondary
taskfragment area

Test: manual - repro steps in bug
Bug: 271592105

Change-Id: I2d724736d01a8fc50fe4c82c8b8cca2769d97d4f