Add Extra to mark ClipData items as "sensitive".

This extra enables applications to mark ClipData items as sensitive,
indicating that the data within should not ordinarily be displayed.

Applications (e.g. IMEs that store clipboard history) can use
this flag to prevent sensitive data from being unintentionally
displayed on screen.

Bug: 195554988
Test: N/A
Change-Id: Ibccd3944842a40d3e5afec2dd99f32eb7a3564db