Switch RecoverableSE over to using RemoteAction.

Now that RemoteAction supports PendingIntent, we should be using it
instead of rolling our own fields.  Continue offering legacy
constructor for some existing unit tests.

Create a notification channel for each remote app that throws at a
client app.  All apps targeting O must now use channels, which give
the user better control over notifications.

Mention in docs that strings should be localized.

Test: builds
Bug: 33749182, 35012253, 34676491
Change-Id: Ic57e32025bc6caf784d3746d9f97f6595b0edb69