Make the length of the delete, clear keys dependent on the text.

Min width matches the other buttons in that col
but max width could be huge
fixes b/5492494 all_source_broad_Android:all_source_project_205189_Android - Android - LQA - MR0 ICS (41 Languages):BG_Calculator_Truncated

Change-Id: Ia9059f9521fbd2293806789f4ec1b745f033e190