Better document the reasons behind the neverallow for tcp/udp sockets.

Test: policy compiles.
Change-Id: Iee386af3be6fc7495addc9300b5628d0fe61c8e9

This starts the switch for HAL policy to the approach where:
* domains which are clients of Foo HAL are associated with
  hal_foo_client attribute,
* domains which offer the Foo HAL service over HwBinder are
  associated with hal_foo_server attribute,
* policy needed by the implementation of Foo HAL service is written
  against the hal_foo attribute. This policy is granted to domains
  which offer the Foo HAL service over HwBinder and, if Foo HAL runs
  in the so-called passthrough mode (inside the process of each
  client), also granted to all domains which are clients of Foo HAL.
  hal_foo is there to avoid duplicating the rules for hal_foo_client
  and hal_foo_server to cover the passthrough/in-process Foo HAL and
  binderized/out-of-process Foo HAL cases.

A benefit of associating all domains which are clients of Foo HAL with
hal_foo (when Foo HAL is in passthrough mode) is that this removes the
need for device-specific policy to be able to reference these domains
directly (in order to add device-specific allow rules). Instead,
device-specific policy only needs to reference hal_foo and should no
longer need to care which particular domains on the device are clients
of Foo HAL. This can be seen in simplification of the rules for
audioserver domain which is a client of Audio HAL whose policy is
being restructured in this commit.

This commit uses Audio HAL as an example to illustrate the approach.
Once this commit lands, other HALs will also be switched to this
approach.

Test: Google Play Music plays back radios
Test: Google Camera records video with sound and that video is then
      successfully played back with sound
Test: YouTube app plays back clips with sound
Test: YouTube in Chrome plays back clips with sound
Bug: 34170079
Change-Id: I2597a046753edef06123f0476c2ee6889fc17f20

This leaves only the existence of audioserver domain as public API.
All other rules are implementation details of this domain's policy
and are thus now private.

Test: No change to policy according to sesearch, except for
      disappearance of all allow rules to do with audioserver_current
      except those created by other domains' allow rules referencing
      audioserver domain from public and vendor policies.
Bug: 31364497

Change-Id: I6662394d8318781de6e3b0c125435b66581363af

Divide policy into public and private components.  This is the first
step in splitting the policy creation for platform and non-platform
policies.  The policy in the public directory will be exported for use
in non-platform policy creation.  Backwards compatibility with it will
be achieved by converting the exported policy into attribute-based
policy when included as part of the non-platform policy and a mapping
file will be maintained to be included with the platform policy that
maps exported attributes of previous versions to the current platform
version.

Eventually we would like to create a clear interface between the
platform and non-platform device components so that the exported policy,
and the need for attributes is minimal.  For now, almost all types and
avrules are left in public.

Test: Tested by building policy and running on device.

Change-Id: Idef796c9ec169259787c3f9d8f423edf4ce27f8c