calculator: allow pasting into an empty display

Long press events were not received by the scrollable display because
HorizontalScrollView implements onTouchEvent and returns true.  To get
around this we can move the context menu handling one layer down to the
linear layout.

Change-Id: I8742230ec712a6af4ddf1c1c612490c64d1edab8
calculator: visual fixes

* make history fill from the bottom
* update targetSdk so that status bar color is applied on L

Change-Id: I91624eb2ec9de78784d19df0417bd053379f383b
Merge branch 'cm' into HEAD