Set the decay timer for all forked apps.

This will cause native memory frees to be less expensive, but it will cost
a bit of PSS that might hang 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).
(cherry picked from commit ab16dd10)

Change-Id: If614a1dd82f0f2e3a9b7080017d82f749a3bc57b