Use SDK 25 to build JUnit instead of core-libart

Some users of the junit target cannot build core-libart/core-oj
due to limitations of their build environment. They must be
using the junit4-target as that is the only JUnit target that
builds against the SDK. As this is being used to replace the
junit4-target it should be built in the same way.

Switching to use the latest version, 25, of the SDK. Cannot use
current as it has dependencies on other targets that are also
not in the affected build environments.

Bug: 30188076
Test: make checkbuild
Change-Id: I674b8a5be800e2d4124c6db423e3261d22d3212e