Rewrite RKP readme in terms of DICE

Update the RKP readme to match contemporary philosophy about the design.
This includes replacing discussion if the obsolete term `BCC` with a
description of the Android Profile for DICE.

The privacy concerns are relaxed to match updates to the HAL which
remove the superencryption of the DICE chain.

Test: n/a
Fix: 281755202
Change-Id: I3a6fd2cd12599c5843b5dce0044eb16c2afbffa2