Use prebuilt nanopb_generator.py.

Use prebuilt nanopb_generator.py to avoid using the incompatible
version of libprotobuf-python on host.

Test: Local build tests.
Bug: 111798740
Change-Id: Iabca07eb13dbab183fbf85cb0dfe04ce33aa660c
(cherry picked from commit fa47daa47d06429a1ba05aada0f7bcec625bea83)
diff --git a/core/config.mk b/core/config.mk
index 58b656e..4226f2f 100644
--- a/core/config.mk
+++ b/core/config.mk
@@ -651,10 +651,7 @@
 BREAKPAD_GENERATE_SYMBOLS := false
 endif
 PROTOC := $(HOST_OUT_EXECUTABLES)/aprotoc$(HOST_EXECUTABLE_SUFFIX)
-NANOPB_SRCS := external/nanopb-c/generator/protoc-gen-nanopb \
-    $(wildcard external/nanopb-c/generator/*.py \
-               external/nanopb-c/generator/google/*.py \
-               external/nanopb-c/generator/proto/*.py)
+NANOPB_SRCS := $(HOST_OUT_EXECUTABLES)/protoc-gen-nanopb
 VTSC := $(HOST_OUT_EXECUTABLES)/vtsc$(HOST_EXECUTABLE_SUFFIX)
 MKBOOTFS := $(HOST_OUT_EXECUTABLES)/mkbootfs$(HOST_EXECUTABLE_SUFFIX)
 MINIGZIP := $(HOST_OUT_EXECUTABLES)/minigzip$(HOST_EXECUTABLE_SUFFIX)