Define shared memory NDK API

Added shared memory NDK API definition, including the following
functions:

*  int ASharedMemory_create(const char *name, size_t size);
*  size_t ASharedMemory_getSize(int fd);
*  int ASharedMemory_setProt(int fd, int prot);

Bug: 5995644
Bug: 30985702
Test: compiles
Change-Id: I4da6741728b7963f0f1d356dd3d9429e706c88a4