Remove duplicate sdk.TestHelper

As part of the work on the new fixture mechanism some of the TestHelper
functionality was moved into the android/fixture.go package. This moves
the rest and removes the now duplicated TestHelper from the sdk
package.

Also removed some unnecessary & operators.

Bug: 181070625
Test: m nothing
Change-Id: Ia09a5d05e4fab3a4e28cf44b2d947a33541e3925