Slow the WG connectivity watchdog when the screen is off

[kasnder]

Problem

WgConnectivityMonitor polls the tunnel's transfer counters every second for as long as the WireGuard tunnel runs — including overnight with the screen off. The monitor is fully passive (counter reads over JNI; it never wakes the radio and holds no wakelock), but each poll still costs a CPU wake, so a healthy idle tunnel burned one wakeup per second between screen-off and deep doze. Its purpose is catching stalls while traffic flows, which overwhelmingly happens with the screen on.

Change

• Poll every 1s while interactive, 15s while the screen is off (WgConnectivityMonitor.pollIntervalMs). The cadence is sampled each iteration from a new isInteractive supplier, backed by WgEgress.currentInteractive (now @Volatile, since the monitor thread reads it).
• Scale the doze-detection threshold with the active interval (isSuspendGap): a gap past one full cycle + 5s margin means doze. This preserves the previous fixed 6s threshold at the 1s cadence and prevents the normal 15s background cycle from being mistaken for a suspension.
• No behaviour change otherwise: stall detection, prod, restart backoff, and onSuspended rebasing are untouched. Background sync still gets stall detection within one 15s cycle.

Testing

• New WgConnectivityMonitorTest: cadence mapping, threshold preserved at 6s on the interactive cadence, threshold scales past the idle cadence without false suspend detection.
• WgConnectivityCheckerTest and WgEgressRecoveryTest pass unmodified.