Permissions: GET_ACCOUNTS permission cleanup

First, getAccounts*() will now return all available accounts depending
on both GET_ACCOUNTS grants and signature matching. This is different
from before where a caller of getAccounts() would need GET_ACCOUNTS to
get any accounts, but if that same caller called getAccountsByType, they
might have gotten back accounts if they shared a signature with the same
developer.

Second, cleaned up some NPEs and javadoc.

This change was motivated by progress on the cts tests.

Change-Id: I2f36226780e074fdf58214b46de3b79d8319ace1