[scudo] On Android, only release when changing M_DECAY_TIME to 0.

There is no reason to make a force release to OS call when enabling
the decay timer. When disabling, it's logical since the intent
is to start using less memory. When enabling, it's not logical
because it usually happens early in the process life cycle and
a caller can do it themselves if necessary.

This is in Android only code right now, so only affects Android
devices.

Author: cferris1000

compiler-rt/lib/scudo/standalone/wrappers_c.inc

@@ -247,16 +247,17 @@ void SCUDO_PREFIX(malloc_postinit)() {
 INTERFACE WEAK int SCUDO_PREFIX(mallopt)(int param, int value) {
   if (param == M_DECAY_TIME) {
     if (SCUDO_ANDROID) {
-      // Before changing the interval, reset the memory usage status by doing a
-      // M_PURGE call so that we can minimize the impact of any unreleased pages
-      // introduced by interval transition.
-      SCUDO_ALLOCATOR.releaseToOS(scudo::ReleaseToOS::Force);
-
       // The values allowed on Android are {-1, 0, 1}. "1" means the longest
       // interval.
       CHECK(value >= -1 && value <= 1);
       if (value == 1)
         value = INT32_MAX;
+      else if (value == 0) {
+        // Before disabling the interval, reset the memory usage status by doing
+        // a M_PURGE call so that we can minimize the impact of any unreleased
+        // pages introduced by interval transition.
+        SCUDO_ALLOCATOR.releaseToOS(scudo::ReleaseToOS::Force);
+      }
     }
 
     SCUDO_ALLOCATOR.setOption(scudo::Option::ReleaseInterval,