Switch to purge all unused memory to kernel.

The new option purges all memory, but could take some extra time.

However, there isn't much memory in the zygote so this should not
incur much extra time during the purge operation.

Bug: 243851006

Test: Verified that the zygote and zygote 64 on oriole reduce the
Test: RSS by about 300K on average.
Change-Id: Ic5eac43813163efb2c049ed280c2f15a89c3fa18