Update JavaDoc for `InputDevice` touchpad sources

As well as reflecting the change to uncaptured touchpad source values
(which are now `SOURCE_MOUSE | SOURCE_TOUCHPAD`), this change
standardizes on the term "touchpad" over "trackpad" (since this is the
term used in the code), and fixes an error related to drawing tablet
sources (which do not have `SOURCE_TOUCHPAD` as previously claimed).

Bug: 245929943
Test: none
Change-Id: I9489b247b5197e0ade275bc9d44dd66536bdb165