Obfuscate account name in account manager logs and dumpsys.

The account name could be PII or email address that should not be
printed in android logs or dumpsys. In account manager output, call
Account.toSafeString() to replace letters/digits in account name with
character 'x'.

Change-Id: Ic1abd1ecda99a309e8973f13bd3650286bbd59e6
Fix: 111937873
Test: atest AccountTest.java