Fix bug #8654490 android:paddingStart doesn't override android:paddingLeft

- in RTL mode only and if you have left/start or right/end at the same time,
the initial left/right padding (coming from the background drawable or from
some explicit definition) was still used.

- now, override the background left/right initial pading by the left/right one
only and only if there is no start/end padding defined at the same time
(because when start/end are defined, we do not care about left/right padding
except the background ones)

Change-Id: Icc6e69c95ace1307b0c5e9673cbdf3b611b62733