a11y: refine sequential focus starting point

Contains a refinement of the focus restore behavior in ViewRootImpl
when accessibility focus is present.

Rather than restoring input focus to a11y focus, we now attempt to
restore to the directional focus from the a11y focus. Additional
testing from TalkBack shows that this slight change in behavior is
desirable.

For example,
Given focus order v1, v2, v3 and No input initial focus.

a11y focus on v2
Up, Left, Shift-tab restores input focus to v1
Down, Right, tab: restores input focus to v3

previously: restores focus to v2

See
go/android-a11y-sequential-focus-starting-point
for more details.

Bug: 393668723
Test: atest AccessibilityRequestFocusTest (cts)
Flag: android.view.accessibility.a11y_sequential_focus_starting_point
Change-Id: I15c13b030aaf7b5b796246b1275cd1ae23d41692