Allow to reset malloc hooks.

This is used to prevent the additional indirection even after heap
profiling has finished, preventing any performance impact on processes
that are not currently being profiled.

Test: m
Test: flash sailfish
Test: try tearing down & re-enabling hooks

Bug: 120186127

Change-Id: Idc5988111a47870d2c093fd6a017b47e65f5616b