Hide APIs not intended to ship in DR.

This CL will be reverted once it merges into the next release branch
to re-introduce the proposed APIs.

This CL also makes the following code changes to preserve API
compatibility:

-- It keeps the recently added RemoteViews.clone() synchronized logic
intact, but moves it inside the method to avoid changing the API
signature.

-- It reverts the RttCall.read() behavior to the oc-dev logic, since
we can't throw IOException until we get an API level bump.  (The
original logic returned null instead of throwing.)

Test: builds, boots
Bug: 62427252, 62431886, 62427329
Bug: 62468911, 62431162, 62428935, 62429096
Bug: 37290820, 37359238, 36886243
Bug: 30143923, 35761231, 62192626, 29829689
Change-Id: I83d723f598cb0ee1fe198e65debd86ef7fd0420c