Extract general bootclasspath related code into java/bootclasspath.go

The platform_bootclasspath and bootclasspath_fragment modules provide
different capabilities but are related and so have some common
functionality. This change moves some platform_bootclasspath code that
will be of use for bootclasspath_fragment in future into a separate
file.

Bug: 177892522
Test: m nothing
Change-Id: I827b85e33d16155fcc920d553100c9e99267dc4e
diff --git a/java/Android.bp b/java/Android.bp
index 8e3e10d..e367a00 100644
--- a/java/Android.bp
+++ b/java/Android.bp
@@ -31,6 +31,7 @@
         "base.go",
         "boot_image.go",
         "boot_jars.go",
+        "bootclasspath.go",
         "builder.go",
         "classpath_fragment.go",
         "device_host_converter.go",