[Android] Enable app cache on Android by default

Currently, app cache flag is enabled by default, but it can't work if
app cache path is not set explicitly. Actually, Chromium doesn't explore
the interface to setting app cache directory.

Crosswalk should not have this limitation, when no app cache is specified,
we should use chromium default cache path instead.

BUG=https://crosswalk-project.org/jira/browse/XWALK-1355
Merge pull request #1816 from yinli/app_cache_enable

[Android] Enable app cache on Android by default