storaged: Don't duplicate uid_records from proto files.

It is possible for storaged to load the same saved proto file multiple
times, for example, if system_server crashes. In this case we do not
want to fill io_history with duplicate entries. This patch elides
records for which an app+userid record already exists for the same
timestamp.

Bug: 111578975
Test: gtest storaged_test.load_uid_io_proto
Change-Id: I87bc3e37d6464079cece03b5852285d79067b935
Merged-In: I87bc3e37d6464079cece03b5852285d79067b935