Add android:backupInForeground

An app can now declare that it really needs to be backed up
whenever possible even if it is currently engaged in foreground-
equivalent work.  Only applies to full-data backup clients: key/value
backups are not intrusive on normal lifecycle so they can already
happen in such circumstances.

Bug 26790411

Change-Id: Ia0ebcc7a53da888ae9ae4d63cd4bcab6e3a2e866