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).
Change-Id: I140b26986f3f81fddd5b9ed9c9e4f2d4977b819b

core/jni/com_android_internal_os_Zygote.cpp

@@ -27,6 +27,7 @@
 #include <fcntl.h>
 #include <grp.h>
 #include <inttypes.h>
+#include <malloc.h>
 #include <mntent.h>
 #include <paths.h>
 #include <signal.h>
@@ -519,6 +520,9 @@ static pid_t ForkAndSpecializeCommon(JNIEnv* env, uid_t uid, gid_t gid, jintArra
     // The child process.
     gMallocLeakZygoteChild = 1;
 
+    // Set the jemalloc decay time to 1.
+    mallopt(M_DECAY_TIME, 1);
+
     // Clean up any descriptors which must be closed immediately
     DetachDescriptors(env, fdsToClose);