Add implementation for PrimaryAccountMutatorImpl [1/N]

Add implementation and test for PrimaryAccountMutatorImpl
methods {Set,Is}SettingPrimaryAccountAllowed.

Bug: 889902, 806778
Change-Id: Ibd75f2b3a988b4c403eab3b682a157e31e4b2e18
Reviewed-on: https://chromium-review.googlesource.com/c/1335937
Reviewed-by: Colin Blundell <blundell@chromium.org>
Reviewed-by: Mihai Sardarescu <msarda@chromium.org>
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#608390}
7 files changed