Cache AVD to improve instrumented tests time #252

Job	Run time
Ensure the code builds (OoniStableFullRelease)	2m 52s
Ensure the code builds (OoniStableFdroidRelease)	2m 52s
Ensure the code builds (DwStableFullRelease)	2m 44s
Ensure the code builds (DwStableFdroidRelease)	2m 41s
Run instrumented tests (29, google_apis)	11m 13s
Archive APKs (OoniStableFullDebug)	3m 12s
Run unit tests	3m 10s
Archive APKs (OoniStableFdroidDebug)	3m 15s
Archive APKs (OoniDevFullDebug)	3m 15s
Archive APKs (OoniDevFullDebugAndroidTest)	3m 36s
Archive APKs (DwStableFullDebug)	3m 10s
Archive APKs (DwStableFdroidDebug)	3m 12s
Archive APKs (DwDevFullDebug)	3m 12s
Upload artifact to Firebase App Distribution	41s
	49m 5s

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