Full-screen user switcher is now scrollable.

When the user increases the font and item size through accessibility
settings, our picker gets its items cut off. This CL fixes it, following
guidance from UX and making the picker scrollable while elevating the
bottom add and cancel buttons to a button bar that extends along the
bottom of the screen, following the figma spec.

Fix: 254788900
Test: verified that the user switcher is scrollable when adding the
maximum number of users to the device. Verified that selecting them
still works. Checked on tablet with landscape and portrait mode as well
with maximum font and UI size and with normal sizes as well.

Change-Id: I2be56e624ac53a87e2c51f91bae1571d6559b3cf