[Android] Separate paste popup implementations

Define a common interface for legacy and floating paste popup menu
implementations. Move existing paste code out from ContentViewCore into
the floating PastePopupMenu implementation.

Also tweak insertion handle tap detection for paste popup dismissal.

BUG=523432

Review URL: https://codereview.chromium.org/1315613005

Cr-Commit-Position: refs/heads/master@{#346430}
12 files changed