feat(android.NearbyTransitPage): leave / rejoin vehicles channel after backgrounding #2841

Job	Run time
Run pre-commit to check formatting and linting	1m 30s
Run Android instrumented tests in emulator (28, default, x86)	8m 2s
Test for Android	5m 16s
Build for Android	3m 31s
Upload to Google Play	0s
	18m 19s

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