API review: change getKeyPairGrants return type

Currently getKeyPairGrants returns Set<Set<String>> which is used
to encode the fact that packages that share UID will always have
access to a given key together. This change makes this UID explicit
by changing return value to Map<Integer, Set<String>>.

Bug: 184018067
Test: atest MixedDeviceOwnerTest#testDelegatedCertInstallerDirectly
Test: atest MixedDeviceOwnerTest#testKeyManagement
Change-Id: I571e67bda1709ddb48012003de055c7211edb786