Change the decay timer default to 0.

Default to always purging, and then assume that people can change
this value if they are willing to leave a bit of PSS hanging around.

Bug: 36401135

Test: Built and booted bullhead.
Test: Ran jemalloc unit tests.
Test: Ran bionic unit tests.
Test: Ran a test that allocated and free'd a large piece of memory,
Test: and verified that after changing the parameter, the PSS
Test: sticks around (decay timer set to 1), the PSS is purged (decay
Test: timer set to 0).
Change-Id: I7819bd8b91a1df600967fa15eebc19fa382d7ab4