The purpose of this change is to add support for allocating gralloc buffers
from either /dev/pmem or /dev/pmem_adsp depending on the usage flags. It does
this by factoring out and abstracting the interactions with the pmem device.
For /dev/pmem allocations, the kernel allocator is not used, so a single master
fd is opened, and all the allocations are sub-allocated from that by gralloc.
For /dev/pmem_adsp the kernel allocator is used, so it simply opens a new fd
for each allocation.
A very basic unit test that can be run on the host is included. It requires
gtest, so to run it on a host system gtest must (currently) be compiled with
BUILD_WITH_ASTL=true.
Change-Id: If2ae0151698fad8107e18e808a3fa012a846263f
