feat(android.NearbyTransitPage): leave / rejoin vehicles channel after backgrounding #2797

Job	Run time
Build for Android	6m 32s
Run Android instrumented tests in emulator (28, default, x86)	10m 32s
Run pre-commit to check formatting and linting	2m 38s
Test for Android	6m 16s
Upload to Google Play	0s
	25m 58s

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