Fix few A11y/ UI bugs for widget pickers

- Add a subtext in pin widget dialog informing users how to add widget
manually. This is not important for accessibility, as drag and drop is through an
action item for visually impaired users.
- Update search bar text hint color so that it passes accessibility test.

Bug: 182994384
Bug: 182993604
Test: Manually Tested using accessibility scanner app and talkback
Change-Id: I5a49d0bc3e5973b76c41b2a178af509e96a3cdd7