LockSettingsService : Restrict access to getpassword API

Restrict Permission of getpassword API to ACCESS_KEYGUARD_SECURE_
STORAGE to ensure saftey for user credentials. No process without
this permission should be able to invoke this API from locksetting
aidl.

CRs-Fixed: 2576302
Change-Id: I7085a00acbdb3e0ea246210207e83c80ab48dc38
Signed-off-by: Prerna Kalla <prernak@codeaurora.org>