Implement skeleton osmigration TargetService aka DataImporterService

This is a gRPC service for importing user data from other browsers,
exposed as an Android service.

The actual service implementation lives in the "on_demand" module, so
that the grpc libraries don't have to be loaded on every browser
startup.

This CL adds the .proto service definition, plus a skeleton
implementation - it responds to all of the RPCs, but doesn't actually do
anything.

Note: This significantly increases binary size due to new third_party
libraries being used (io_grpc_grpc_*), which has been approved (see
https://groups.google.com/a/google.com/g/chrome-atls-discuss/c/WXAcDL2zCdQ).
The bulk of the increase is in on_demand.apk, so will not slow down
browser startup.

Binary-Size: Size increase is unavoidable (see above).
Bug: 431218724
Change-Id: I4baf87133a0ff6ee392782642c4449601cd81466
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6760874
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Reviewed-by: Filipa Senra <fsenra@google.com>
Commit-Queue: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1490049}