[Android] Catch missing xwalk library make_apk.py

When creating (x86/ARM) packages, emit a warning if the
corresponding architecture libxwalkcore.so is not found.
If no packages have been created due to missing
libxwalkcore.so, abort with an error message.
Merge pull request #1813 from r0b5t4/wip

[Android] Catch missing libxwalkcore.so in make_apk.py

When Crosswalk is built as a shared component, libxwalcore gets a
.cr.so suffix, and make_apk silently bails, not finding it. This patch
checks for the existance of libxwalkcore.so, and prints an
understandable error when it is not there.