[Android] Enable Chromium command line support

Add the mechanism to support the Chromium command line in xwalk.
1 Read commands from the command line file which locates in assets
in xwalk core.
2 Expose the option to set the Chromium command line in make_apk.py.

BUG=XWALK-1269
Merge pull request #1739 from xingnan/command-line

[Android] Enable Chromium command line support