Implement Safemode alarm in VcnGatewayConnection.

This CL updates VcnGatewayConnection to use WakeupMessages to support
Safemode detection. Safemode is entered if the VcnGatewayConnection
instance takes more than SAFEMODE_TIMEOUT_SECONDS seconds to a) go from
DisconnectedState to ConnectedState, or b) exit and re-enter
ConnectedState. If this timeout is exceeded, the instance will notify
its managing Vcn via VcnGatewayStatusCallback#onEnteredSafemode.

Bug: 178140973
Test: atest FrameworksVcnTests
Change-Id: Iff40aba639b465c3959ef383f1001be419daba4d