Make the IsSdkVariant field and method the same.

Before, the IsSdkVariant() method also checked AlwaysSdk(). Instead,
sdkMutator now sets the IsSdkVariant field to True even for the single
variation it creates when AlwaysSdk is true.

Test: Presubmits.
Change-Id: Ied8b42a3441a30484ef8c5dff5aade5fe92574bb