launcher: create attrs for more horizontal margin options

According to new specs, margins are different depending on orientation
and fold/unfolded. Also change the name of the variable to
horizontalMargin, since it applies the same padding to both sides.

Bug: 191879424
Test: manual and checking paddings in HSV
Change-Id: I8990c02b928a22b9f3b0cf3cbc3abf08df4daa9d