Update tablet keyboard layout to the latest design

This change introduces shiftedIcon attribute for Key which specifies
icon to draw the shift key while the key is in shifted state.

This change also intriduces new configurable dimension value
key_label_horizontal_alignment_padding which represents horizontal
padding for left or right alignment of key label.

Bug: 3216592
Bug: 3201839
Change-Id: I3aa7392227ce573c404517678e6340a8d1fcb328