Add gRPC API implementation

Changes:
- Rename the existing daemon to applaunchd-dbus, and add a new
  version that exposes a gRPC based implementation of the API based
  on the protobuf definition added in a top-level protos directory.
  Having both the D-Bus and gRPC APIs exposed by a single daemon was
  posing difficulties around startup dependencies stemming from D-Bus
  activation of the daemon.  Since the end goal is dropping the D-Bus
  API entirely, it is easier to just add a second daemon for gRPC that
  will eventually be the only one present.
- To facilitate building the two implementations, a significant amount
  of code refactoring has been done to move things from the D-Bus API
  implementing app_launcher.[ch] to systemd_manager.[ch] so the code
  can be reused.
- All use of the systemd D-Bus library except for the path encoding
  helper function has been replaced with GDBus.  The systemd interface
  wrapper code was generated with gdbus-codegen from XML files captured
  via introspection on a running system.  The motivation for this
  change was to avoid multithreading issues with sd_bus exposed when
  calling into it from the gRPC threads.
- The copyright headers in the source files have been tweaked to
  remove the Apache license boilerplate in favour of a SPDX license
  tag.

Notes:
- The gRPC API differs slightly from the D-Bus one in that it has a
  single status streaming RPC method as opposed to the separate signals
  for application started or terminated that the D-Bus API has.
- The gRPC API is currently unauthenticated, the aim is to circle back
  and implement authentication once a consensus can be reached on what
  mechanism should be used (fixed JWT configuration, OAuth, etc.).

Bug-AGL: SPEC-4559

Signed-off-by: Scott Murray <scott.murray@konsulko.com>
Change-Id: I828f38a58b60e9959162b98054982124d4fa4380

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