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

google3 callsites are updated, remove global lookup from constructor parameters.

Add feature flags for android, ios, and fuchsia (on by default). After updating the g3 rollers, the fuchsia feature will be turned off by default. Creates a simpler base type of feature flags for g3 to extend.

Updates android, ios, fuchsia workflows to use feature flags check.
Removes concept of stable artifacts and checks on flutter version.

Fixes #58999
#52859
#12768

This reverts commit 602d8baf.

This reverts commit 9186dfc3.

This reverts commit 1bf9d6f4.