Always save long screenshot temp file to the same place

If we fail to clean up the file, they won't accumulate over time.

Bug: 191499234
Test: adb shell ls /data/user_de/0/com.android.systemui/cache/long_screenshot_cache* to view files
Test: Verify state restoration still works
Test: Verify temp file disappears when activity is done.
Test: Verify that temp file is overwritten when a new session begins.
Change-Id: Ie15065ef9ff3db91852df1a0dc96afd494e85b8c