[Android] Make enhanced bookmark editing UI more straightforward.
- We didn't save when a dialog is dismissed by tapping outside on
tablet, which can be also confusing as there is no save UI concept.
So save onPause always.
- User should know when we won't save what user typed if it fails
validation. But blocking dialog exit is too invasive, and sometimes
we don't have a control (tapping outside on tablet.) So show warning
as user types.
BUG=510253
Review URL: https://codereview.chromium.org/
1230313009
Cr-Commit-Position: refs/heads/master@{#342719}