sdkmanager: Default preview tools to true

We change the default setting to show preview tools.
The settings key for this property has also been changed
so that all users get reset to this new setting that defaults
to true, regardless of whether they have enabled or disabled
it in the past.

Change-Id: I8e90951b119c9a65e754823a5e9d9f60087f655a
3 files changed