Initial work for key rotation.

Introduces the upgrade-keyset tag to AndroidManifest.xml. This specifies a
KeySet by which an apk must be signed in order to update the app. Multiple
upgrade KeySets may be specified, in which case one of them must be used to
sign the updating apk.  If no upgrade-keyset is specified, the current logic
involving signatures is used.

Current Key Rotation Design Decisions:
-Apps using a shared user id may not rotate keys.
-All acceptable upgrade keysets must be specified, including the key signing
the app.  This enables key rotation in one update, but also 'locks' an app if
an incorrect upgrade keyset is specified.
-Minimal changes to existing KeySet code.

Bug: 6967056
Change-Id: Ib9bb693d4e9ea1aec375291ecdc182554890d29c