Handle process die event of a bubble activity gracefully.

When the process behind a bubble activity died (e.g. killed by lmkd,
or app crash), collapse the bubble and restart the activity when it's
expanded next time.

Bug: 126945401
Test: atest TaskStackChangedListenerTest
Change-Id: Ia2ee21f9ced51fe33756b1e12159f445d6ed083d