Create recents protectionLevel

In preparation for permission BROADCAST_CLOSE_SYSTEM_DIALOGS
(go/close-system-dialogs), which will be signature|recents in order to
lock down Intent.ACTION_CLOSE_SYSTEM_DIALOGS. That intent is used by
recents when it's invoked.

Currently, recents is configured via existing config
config_recentsComponentName, so using that to determine the recents
package in PM.

It's my understanding that the package defined in such config will be
responsible for recents no matter which launcher is active. It's also my
understanding that the package being explicit in the system config means
that package has to come from the system image, hence it's fine to use
ensureSystemPackageName() in PM. Please advise if any of
this is wrong.

Test: Will work on CTS once we agree on the proposal here.
Bug: 159105552
Change-Id: I9f73e1f05d8df26666da156b672e370dce5030de