add platform level in README

N/A

Change-Id: I049853039da9adf10ec32e9c2f68e2ede6475e5c
Signed-off-by: mochangming <mochangming@xiaomi.com>
make keyguard related code being independent of framework/base

Change-Id: I922c2949922acd0c108c5b6ee9eb5090114b47c1