Suppress horizontal scrolling with trailing blanks

The existing behavior of EditText is that trailing blanks can cause a
line to exceed the layout width, causing the cursor to extend past the
line, which in turn causes horizontal scrolling. This patch clamps the
cursor to the layout width in the non-scrolling case, which makes the
spaces effectively invisible when they're at the end of the line, but at
least suppresses the scrolling.

The clamping only works reliably in left-to-right alignments, so this
patch checks for than and only enables the clamping in those cases.

Fix for bug 7699295.

Change-Id: I22bc4e6c9ded3d7716edfcf10dd2b5c31a5da9de