Add support for modifying decay timer.

Add the mallopt function, and only a single option so far.

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: I6927929b0c539c1023d34772d9e26bb6a8a45877