Stop using versioned snapshots in TestBasicSdkWithBootclasspathFragment

In preparation for removing support for versioned snapshots from Soong
this change rewrites TestBasicSdkWithBootclasspathFragment to use
non-versioned prebuilts making it more realistic in the process.

Bug: 260237150
Test: m nothing
Change-Id: I99d1ad9b480657bb805c3d48e99e715052f4d00a