Release screenshot WindowContext when process ends

Bug: 176600157
Fix: 176600157
Test: manual -- reproducible by taking a screenshot several times
(5-10) in quick succession, and observing crash in logs. Crash no
longer occurs when we release the WindowContext when the screenshot
process is destroyed.

Change-Id: I33047ffb03867f65b591a1b43f6a56d49093fc0e