Ensure that LockSettingsService waits for gatekeeperd to be ready

gatekeeperd is started asynchronously.  Thus, there is no hard guarantee
that it will be ready when LockSettingsService first tries to use it.
Therefore, use waitForService() to wait for it to be ready.  To avoid
introducing any unnecessary delay, stop calling getGateKeeperService()
from LockSettingsService#systemReady(), where GK isn't yet needed.

This is motivated by a bug that caused gatekeeperd to wait 5 seconds
during start-up, fixed by https://r.android.com/2623413.  Apparently,
that was enough of a delay to sometimes cause gatekeeperd to not be
ready by LockSettingsService#loadEscrowData().

Bug: 283055837
Test: Boot and 'atest com.android.server.locksettings'
Change-Id: I815151cc6024c447bf3eff194a911ecdaab22d10