Factor out a class to store the rebootEscrow key

To support server based RoR, we will communicate with server to
encrypt the time constrained reboot escrow key; and put the
encrypted key on /data. This replaces the call to the reboot
escrow HAL. Add an interface to support both implementations.

Bug: 172780686
Test: atest FrameworksServicesTests:RebootEscrowDataTest \
            FrameworksServicesTests:LockSettingsServiceTests \
            FrameworksServicesTests:RecoverySystemServiceTest \
            FrameworksServicesTests:RebootEscrowManagerTests
Change-Id: Icf10d5771b1efd8197b677158f39fd240c5c4107