bluetooth: Move mock creation out of BluetoothDispatcherHost to
LayoutTestBluetoothAdapterProvider.

This patch doesn't add any new tests.

Split for easier reviewing. This is the last of two patches to remove
testing from BluetoothDispatcherHost and BluetoothDispatcher:

  [1] http://crrev.com/1125133005
  [2] This patch.

BUG=436284

Review URL: https://codereview.chromium.org/1132943002

Cr-Commit-Position: refs/heads/master@{#330748}
12 files changed