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This change extracts all caBLEv2 bits from the caBLEv1 discovery and
puts them in a separate class. This class is not an active discovery: it
depends on BLE adverts getting fed to it from the caBLEv1 discovery. But
this style permits the caBLEv2 discovery to be used in unittests because
the BLE adverts can be fed from anywhere.

Since caBLE only has a single transport value, this means that the
discovery factory can now return a vector of discoveries for a given
transport. It might be the case that we should split the transport
value, but since the v2 discovery depends on the v1 discovery, that's
not as simple as it sounds.

BUG=1002262

Change-Id: Ia3c277e8f7211beb81d352a1cb31918dc643abbe
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2415430
Commit-Queue: Adam Langley <agl@chromium.org>
Reviewed-by: Martin Kreichgauer <martinkr@google.com>
Cr-Commit-Position: refs/heads/master@{#808709}