[Android] Stop temporarily hiding selection handles

[Android] Stop temporarily hiding selection handles

Previously, selection handles would be hidden when the content view
became unfocused. This approach worked fine in Chrome, where the focus
model is predictable, but led to undesirable consequences in WebView,
e.g., the handles never being shown. For now, avoid making any kind of
visibility changes to the handles when the focus changes, ensuring
handle visibility is consistent with the selection.

Also ensure timely cleanup of the PopupTouchHandleDrawable Java object
after its native counterpart has been destroyed.

BUG=430859
(internal b/18066784)

Review URL: https://codereview.chromium.org/684103006

Cr-Commit-Position: refs/heads/master@{#303728}