KeyChain: Improve key override documentation.

Improve the documentation on the case of key override: When a new key is
generated/installed using an alias that already exists.

In particular, clarify that grants are lost and that a new call to
KeyChain.choosePrivateKeyAlias must be issued in this case.

Bug: 123563258
Test: that it builds.
Change-Id: I055e95f57b9576883736ca0cfa6a998dec08a6c2