Make memfd switch optional

[pundiramit]

Some devices (e.g. beryllium) still prefer to stick to Ashmem for now, mostly to keep selinux in enforcing mode.

[pundiramit]

@amartinz @calebccff @vknecht @lweiss-fairphone please smoke test on your respective devices and let me know your thoughts on this one.

[z3ntu]

(@lweiss-fairphone is my other account btw) I've also patched ashmem back into my kernel, to keep selinux enforcing; and my device is anyways not supported in this branch

[amartinz]

axolotl does not work properly when using memfd and i had to disable it locally to get the version for the demo up and running.

That's why i would prefer if axolotl would stick to ashmem as well, especially because of selinux.

I think we should revert memfd for all devices and reenable once we have all the pieces (including proper sepolicies) in place?

[calebccff]

I've been using android-mainline with ashmem, so i don't really mind. reverting seems sensible for enchilada too right now

[amartinz]

In any case, smoke tested with axolotl and it works:

axolotl:/ $ getprop | grep memfd
[sys.use_memfd]: [true]
axolotl:/ $ getenforce                                                                                                                                                                                                                                                                                                       
Permissive

(seems memfd issues are gone now and i just had a bad sync last time).