Improve the structure of NNAPI AIDL Memory

Prior to this change, the NN AIDL HAL created a Memory struct analogous
to hidl_memory, consisting of a name, size, and native handle. However,
this Memory struct is not very structured, and requires both the client
and server to agree on how the data should be interpreted.

This CL tightens the structure of the Memory representation by
introducing Ashmem and MappableFile structs to android.hardware.common
in order to hold specific file descriptors representing memory regions.
Further, this CL redefines android.hardwre.neuralnetworks's Memory
object as a union of the Ashmem, MappableFile, and (existing)
HardwareBuffer memory types.

This change also adds "com.android.neuralnetworks" to the graphics
AIDL HAL's apex_available build rule.

Bug: 183118727
Test: mma
Change-Id: I32322df676b83597c9e95f13662c322a6a80accc