Copy KeyStore.UID_SELF to KeyProperties.UID_SELF

To make it possible to remove the obsolete and misleadingly named helper
class android.security.KeyStore, the UID_SELF constant will be moved to
KeyProperties.  This CL completes the first half of the move by adding
the constant to the new location.  A later CL will delete the constant
from the old location once all users have been updated.

Bug: 326508120
Test: build
Flag: NONE mechanical refactoring with no behavior change
Change-Id: I5aa839a19d548d9d5a86afc30ab39431c0b93aa5