ART: Minor refactoring of JNI stub compilation.

Introduce JniCompiledMethod to avoid JNI compiler dependency
on CompiledMethod. This is done in preparation for compiling
JNI stubs in JIT as the CompiledMethod class should be used
exclusively for AOT compilation.

Test: m test-art-host-gtest
Bug: 65574695
Change-Id: I1d047d4aebc55057efb7ed3d39ea65600f5fb6ab