Fix leanback grid view addFocusables() when isFocusable is false

When strategy is FOCUS_SCROLL_ALIGNED, we need add the last focused
item remembered at mFocusPosition into the focusables list, so even
the GridView itself is not focusable, we could make FocusFinder
selects the child item.

Bug 26918143
Needed to fix Bug 26894680

Change-Id: I7125ea0389a0299deb7d0fd31c02a5c3dc77e74f
3 files changed