Add MALLOC_LOW_MEMORY as synonym for MALLOC_SVELTE.

The MALLOC_SVELTE variable name is confusing, so add a new variable
name called MALLOC_LOW_MEMORY. Still support MALLOC_SVELTE.
Also, support the change of Malloc_not_svelte to Malloc_low_memory.

Test: Verified scudo config is used by default.
Test: Verfified Android GO config uses the jemalloc low memory config.
Change-Id: I2606205da746d8ab20e29884c7dcb5a77add51e4