TestRule to set Navigation via "Buttons" or "Gesture"

[sergio-sastre]

Describe the feature
Beginning with Android 10 (API level 29), the Android system supports fully gesture-based navigation, as stated here:
https://developer.android.com/develop/ui/views/touch-and-input/gestures/gesturenav

These can be set via adb, and therefore you can create a TestRule for them
ndtp/android-testify#237 (comment)

Enable 3-button navigation and disable gesture navigation:

adb shell cmd overlay enable com.android.internal.systemui.navbar.threebutton
adb shell cmd overlay disable com.android.internal.systemui.navbar.gestural

Enable gesture navigation and disable 3-button navigation:

adb shell cmd overlay enable com.android.internal.systemui.navbar.gestural
adb shell cmd overlay disable com.android.internal.systemui.navbar.threebutton