[flutter_tools] reland: avoid creating Android Devices if AndroidSDK cannot be found (#64665)

Avoid creating AndroidDevice discovery if the SDK cannot be located. Previously the tool would use which/where adb, however this required us to handle the AndroidSdk class being potentially null - which required an additional layer of indirection around all access. Sometimes these were forgotten leading to NPEs.

In general, not much can be done with an Android Device if the actual SDK is not installed.

Reland with fixed code + tests for null SDK + adb in AndroidDeviceDiscovery