UserManager.getUserName w/ GET_ACCOUNTS_PRIVILEGED

Previously, UserManager.getUserName() and getUserIcon()
required the MANAGE_USERS permission; this has been relaxed
so that the GET_ACCOUNTS_PRIVILEGED permission is also sufficient.

Test: atest UserManagerServiceUserInfoTest
Fixes: 127826840

Change-Id: If90b82313ecf23ea56dca3d24f23a4ca9caa939a