Tweak GET_ACCOUNTS behavior and improve memory.

Related to recent permissions and system health changes. This change
will make it so that calls to AccountManager#getAccountsByType will work
for the owning account authenticator even if they don't have
permissions. This is pretty fundamental to having a working
authenticator and it doesn't make sense to have it be disabled (or have
authenticators hack around the framework).

Also changed how TokenCache works so that memory usage is still
predictable (no more than 64kb) but token caching won't be at the mercy
of garbage collection. This is important for writing stable cts tests.

Change-Id: Ib31b550616b266ee5a04eb26b04ba0023ca0cb83