Allow drm hal implementations to not require provisioning

Not all drm schemes require provisioning. This change
updates the documentation to explicitly support it.

bug:36007160
Change-Id: Ia4dc471314d2cce92ce8d69258bf1d16d5104302
diff --git a/drm/1.0/IDrmPlugin.hal b/drm/1.0/IDrmPlugin.hal
index 5bae22d..083b311 100644
--- a/drm/1.0/IDrmPlugin.hal
+++ b/drm/1.0/IDrmPlugin.hal
@@ -179,8 +179,9 @@
      * certificate authority (CA) is an entity which issues digital certificates
      * for use by other parties. It is an example of a trusted third party.
      * @return status the status of the call. The status must be OK or one of
-     * the following errors: BAD_VALUE if the sessionId is invalid or
-     * ERROR_DRM_INVALID_STATE if the HAL is in a state where the provision
+     * the following errors: BAD_VALUE if the sessionId is invalid,
+     * ERROR_DRM_CANNOT_HANDLE if the drm scheme does not require provisioning
+     * or ERROR_DRM_INVALID_STATE if the HAL is in a state where the provision
      * request cannot be generated.
      * @return request if successful the opaque certificate request blob
      * is returned