Clear calling identity before calling mountService

Change I8106bfba28da401b9fd38349c6a9fa9a24f54712 unwittingly
reintroduced requirement for apps that change password to have
STORAGE_INTERNAL permission.
This fixes the problem by clearing calling identity before
calling mountServce in fixateNewestUserKeyAuth, like it's done
in addUserKeyAuth.

BUG=28154455
BUG=27600227

Change-Id: I1bcd9f6eefa892b021d93df2638c8dba4d4b5400