Add support for LOCAL_GENERATED_SOURCES to droiddoc

BUILD_DROIDDOC allows LOCAL_SRC_FILES to refer to sources in
$(LOCAL_PATH) and LOCAL_INTERMEDIATE_SOURCES to refer to sources
in $(TARGET_OUT_COMMON_INTERMEDIATES).  The droiddoc rules that
use files generated in Soong were trying to abuse LOCAL_SRC_FILES
to point to files in $(OUT_DIR)/soong using ../.. to counter
$(LOCAL_PATH), but failed if $(OUT_DIR) was absolute.  Add
LOCAL_GENERATED_SOURCES that can take a path relative to the
top of the source tree or an absolute path.

Bug: 70166718
Test: m OUT_DIR=/tmp/out docs
Change-Id: Iaf0d622a3405da0bf520f7312e11e75e9328062e