Introduce derive_classpath.

The service generates /data/system/environ/classpath with values for
BOOTCLASSPATH, SYSTEMSERVERCLASSPATH, and DEX2OATCLASSPATH to be
exported by init.

See go/updatable-classpath for more details.

Bug: 180105615
Test: manual
Change-Id: I4150de69f7d39f685a202eb4f86c27b661f808dc
diff --git a/apex/com.android.sdkext-file_contexts b/apex/com.android.sdkext-file_contexts
index 2d59dda..551a12c 100644
--- a/apex/com.android.sdkext-file_contexts
+++ b/apex/com.android.sdkext-file_contexts
@@ -1,2 +1,3 @@
-(/.*)?                u:object_r:system_file:s0
-/bin/derive_sdk       u:object_r:derive_sdk_exec:s0
+(/.*)?                       u:object_r:system_file:s0
+/bin/derive_classpath        u:object_r:derive_classpath_exec:s0
+/bin/derive_sdk              u:object_r:derive_sdk_exec:s0