Add unit test to verify contents of bootclasspath.pb.textproto

The unit test is useful for verifying that
min_sdk_version/max_sdk_version information is populated correctly.
Followup to https://r.android.com/3128893

Bug: 345621958
Test: go test ./apex
Change-Id: Id7696a8cefaab6d0b7f38b65a33e326152ac12d3