Expire landing flags after an hour.

To do this, factor out the TTL code from Spark into Fuse, a
new class that implements Removable by burning down the
given fuse to 0f.

Fixes: 345812036
Flag: com.android.egg.flags.flag_flag
Test: adb shell am start -n com.android.egg/.landroid.MainActivity
Test: manual instructions: let the screensaver run for a while
Change-Id: Ibb1d2d67d0b9ce577ce41224b9f061577bd5c018