graphics: clarify importBuffer and passthrough HALs

A buffer handle recieved from a HAL is by definition raw and needs
to be imported.  But because of passthrough HALs, such a raw handle
may have been imported already.  Explicitly specify that an
implementation must accept such a raw handle.

Bug: 37540361
Test: boots on angler, ryu and marlin
Change-Id: I5ecf526e59b27cc4a8f7f5d5ec27477da0946ece
diff --git a/graphics/mapper/2.0/default/GrallocMapper.cpp b/graphics/mapper/2.0/default/GrallocMapper.cpp
index 6441af6..d16143d 100644
--- a/graphics/mapper/2.0/default/GrallocMapper.cpp
+++ b/graphics/mapper/2.0/default/GrallocMapper.cpp
@@ -125,11 +125,8 @@
 
 Return<void> GrallocMapper::importBuffer(const hidl_handle& rawHandle,
                                          importBuffer_cb hidl_cb) {
-    // importing an already imported handle rather than a raw handle
-    if (gRegisteredHandles->get(rawHandle.getNativeHandle())) {
-        hidl_cb(Error::BAD_BUFFER, nullptr);
-        return Void();
-    }
+    // because of passthrough HALs, we must not generate an error when
+    // rawHandle has been imported
 
     if (!rawHandle.getNativeHandle()) {
         hidl_cb(Error::BAD_BUFFER, nullptr);