embedded.mk: allocator service

We are splitting android.hidl.memory into android.hidl.allocator and
android.hidl.memory to reflect the fact that we have two separate
interfaces which are served over different transports.

Bug: 35327976
Test: hidl_test, device boots with allocator
Change-Id: I36b1554d20f89ecd60a836c04f788ac83e0a1f5c