Permit plugin-x plugins to ship third-party libraries.

Ignoring the libs directory's .jar files is strange. I'm not sure
whoever put the total block over libs/ knew that this is the standard
Android library directory and not just for NDK-compiled assets.

More granular ignores over libs/ contents would be better in the
future, but this fixes the odd sdk/ directory hack plugins currently
use and lets the Android build system auto-link libraries that're
shipped with plugins instead of forcing Cocos developers to manually
sift through a plugin's Android dependencies and include them by hand.
Merge pull request #9108 from nilium/v3-plugin-libjars-unignore

Permit plugin-x plugins to ship third-party libraries.