[xpconanfan]

adb root exits 0 even when adbd refuses to restart as root (e.g. "adbd cannot run as root in production builds"; the exact message varies across OEMs), so Mobly's automatic adb root on debuggable builds could fail without any indication. Separately, there was no way to opt out of automatic rooting, which matters for tests that must behave identically on user and userdebug builds (e.g. CTS).

This PR:

• AndroidDevice.root_adb() now verifies the outcome via is_adb_root (id -u == 0) after adb root, logs a warning if it did not take effect, and returns a bool. No new exception path, so devices that report ro.debuggable=1 but refuse root keep working as before, just no longer silently.
• Adds a skip_root option (constructor kwarg + skip_root controller config key, mirroring skip_logcat) that disables automatic adb root at creation time and after handle_reboot. Explicit root_adb() calls are unaffected.

Automatic rooting still happens in __init__, so code that constructs AndroidDevice(serial) directly keeps its current behavior.

Fixes b/419425573 (internal).

[xpconanfan]

there should be a debug/verbose log here so we know what happened

[xpconanfan]

Added debug logs when skipping automatic adb root (both when root_on_init is "never" and when the build is not rootable), and moved _root_adb_on_init() out of AndroidDevice.__init__ into get_instances / get_instances_with_configs (with root_on_init as a validated settable property loaded via load_config).