fix(focus): claim and directional search fixes

[RuudBurger]

Four FocusManager fixes from running the Plex Lightning app on native-style focus (on top of the claim patience in #133).

• Empty redirect-only groups (a 0x0 claim region) are skipped when a focused item unmounts and when entering a group. An uncached show page's remounting button used to leave focus on the invisible claim region.
• A removed claim no longer blocks the next one. A later p=10 claim never took over once the p=5 target had gone.
• Directional moves beam from the focused leaf, like tvOS and Android. Up from Activity on a details page went to the summary instead of the action bar (and to Restart instead of Resume on a movie). Falls back to the row when nothing lines up with the leaf.
• A claim's own fallback landing doesn't count as last-focused, so ['last-focused', episodeKey] moves to the episode when the key shows up a few ms late instead of staying on its fallback.

Caveat: the last one means a later autoFocus child can still take the slot in a claimed region until the user moves in it. Seems right to me, since a claim landing is a mount-time default, but shout if that bites.

Tests: 382 pass, 5 new regression specs (all failed before). One patch changeset per fix.