Add explicit memfd support to MemoryHeapBase

The default behavior of MemoryHeapBase is to request memory from
ashmem in libcutils. This library is being used to migrate from
ashmem to memfd, but on supported devices, memfd is still debug
only in order to sanitize expectations of callees of the library.

As such, we add a flag to explicitly request memfd files, as well
as to opt into file sealing of memfd files.

Note: the calling process will have to modify selinux permissions
to allow tmpfs access to the domains accessing the shared buffer.
This is more secure than ashmem as granular acess can be specified.
See audioserver_tmpfs in system/selinux for examples.

Test: Expected mappings recieved in Audioflinger for flag combos.
      atest binderMemoryHeapBaseUnitTest
Change-Id: I893f7846f4d16b8ccc7f407fb957dccd9ebe62d7