use EXTRA_TEXT when receiving text
throw an exception on null data. display a toast.
Revert "throw an exception on null data. display a toast."

This reverts commit 12ce60a62814d98625a91f4d44049b9f02694de3.
use .equals for string compare
encode text and subject
handle the custom mime type at the right place
handle the mime type at the receiver
restore the text/plain mime type
merged MainActivity