Replace Malloc_not_svelte with Malloc_low_memory.

The Malloc_not_svelte variable name is confusing and makes the
low memory config the default. Change this so that the default is
the regular allocator, and that Malloc_low_memory is used to enable
the low memory allocator.

Test: Verified scudo config is used by default.
Test: Verfified Android GO config uses the jemalloc low memory config.
Change-Id: I02ee2bd304c8367152108caee08dd27d91838659