Fixes & updates to preference support lib

a) Make PreferenceManager.getContext() public, so that callers
   that instantiate Preference instances manually can pass the
   right context
b) Use the internal android id for the widget frame, since
   otherwise, callers that specify a custom preference layout
   will have no way of referencing the right widget_frame id
   in their layout XML

b/22043727
b/22045650
b/22044940

Change-Id: Ib4919e8da01b27bd9c76fcde554d60ab311d7444
5 files changed