Make PackageManager unpack gdbserver binaries at installation time.

Native-debuggable packages contain a lib/<abi>/gdbserver executable.
This patch ensures that the package manager will copy it to the
proper location (/data/data/<appname>/lib) at installation time.

Note that such packages are marked with a new ApplicationInfo flag
named FLAG_NATIVE_DEBUGGABLE, to be used later by the Activity
Manager.