Make cocos' Android.mk self containing (#18392)

[rynkowsg]

PROBLEM

cocos makefile is not self-containing.

From the perspective of a developer that link his target with
cocos2dx_static library, it should specify in his Android.mk only
a path to a makefile that defines that module, by

a) using import-add-path and import-module

$(call import-add-path, <path-to-cocos-root>)
$(call import-module, cocos)

or ...

b) using directly an include command

include <path-to-cocos-root>/cocos/Android.mk

Unfortunately the current configuration of cocos' Android.mk doesn't
specify (using import-add-path) locations of modules that it uses.
In result, every dev that uses cocos to create Android app has to
populate NDK_MODULE_PATH at his Android.mk files by adding two lines:

$(call import-add-path, <path-to-cocos-root>/cocos)
$(call import-add-path, <path-to-cocos-root>/external)

Those lines have to be placed at cocos/Android.mk file.

The current configuration of build script breaks a good practice of
hiding internal details from clients of library or component. In fact
the script exposes internal dependencies and require from a dev to make
them out.

SOLUTION

This commit adds corresponding import-add-path commands to Android.mk
files.

HOW TO TEST

1. Create cocos2d-x app.

2. Import in your Android.mk file a cocos module using one of
   aforementioned approaches.

3. The app should compile without a error message like the one below:

   Android NDK: jni/../../cocos2d/cocos/Android.mk: Cannot find module with tag 'freetype2/prebuilt/android' in import path
   Android NDK: Are you sure your NDK_MODULE_PATH variable is properly defined ?

Issue: #18392