[Android] Reduce the time consumption for make_apk test (2).

Fake an empty native library "libxwalkcore.so" in make_apk test
for embedded mode. It would greatly reduce the time consumption
caused by copying library when generating the apk in embedded mode.
Merge pull request #1279 from junmin-zhu/fakeso

[Android] Reduce the time consumption for make_apk test (2).