Fix bugs around overlay action modes.

Multiple focusable windows cause undesired behavior around selection
modes. TextView isn't sure how to behave when it loses window focus
with regard to selection handles and action modes need to be focusable
for WebView find on page since it uses an EditText as a custom view.

For now:

* Use a layered window decor for overlay action mode when there is no
  action bar requested. This eliminates an extra window and avoids the
  issue described for full-screen UIs.

* Disable WebView's find-on-page mode when the action mode's UI will
  not be focusable. This only affects WebViews in floating windows.

Also remove the "Text Selection" title for WebView's selection mode at
UX's request, as it is inconsistent with TextView's selection mode and
the string does not fit on phones in portrait even on wide
devices. This now uses the same mechanism used in TextView to decide
whether to use title text.

Change-Id: I80caeecea9b47728cf26bb0a388153ca0bdeafe1