Fix SnapshotTaskTests

Added enable_recents_screenshot to windowmanager proto file so it can be
used when testing. The test will read back this value to make sure it's
set before proceeding with taking a task snapshot.

Test: atest SnapshotTaskTests --rerun-until-failure 10
Fixes: 219750596
Change-Id: Ibd66c5f4d46e5102dbe0763f7b0b7cbcfb90a798