[MFill Android] Wire manual generation

This CL adds the missing logic for manual generation, wiring the button
through the generation controller.

Notable changes:
- PasswordGenerationController now uses field focus events to remember
the active frame driver and to reset its own state.
- All generation operations are restricted to the active frame.
- The generation modal dialog is always hidden when a focus change
occurs.

Bug: 835234

Change-Id: Ie839e501d3850b4b6fc8741af480ad289d055528
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1617346
Reviewed-by: Tommy Martino <[email protected]>
Reviewed-by: Vasilii Sukhanov <[email protected]>
Reviewed-by: Friedrich [CET] <[email protected]>
Commit-Queue: Ioana Pandele <[email protected]>
Cr-Commit-Position: refs/heads/master@{#667925}
26 files changed