Increase retention period of staged sessions

The retention period is increased from 7 to 21 days to capture
information about the previous train installation when a rollback occurs
and handle cases where the bug report is taken after a few days of
rollback.

Bug: 208825408
Test: adb shell dumpsys package
Change-Id: I3b0827b59b527fb8a3d0f0b902f85af4ef2c90d4