[Android] Add arch option for packaging tool in embedded mode.

Accept the arch option from developer for embedded mode. if the arch
option is unspecified, all available platform apks will be generated.

Usage: python make_apk.py --arch=x86 ...

BUG=https://crosswalk-project.org/jira/browse/XWALK-598
Merge pull request #1253 from junmin-zhu/arch

[Android] Add arch option for packaging tool in embedded mode.