Fix installation of NPRuntime plug-ins.

Fix installation of NPRuntime plug-ins.

[Issue#] N/A
[Problem] All NPRuntime plug-ins are installed with a widget instead of
only those that match target architecture.
See "Tizen Web Runtime Core Specification 2.1", section
"Web Application Installation", requirements 0102 and 0103.
[Cause] N/A
[Solution] Install from widget package from sub-directory
"plugins/" only those files that are in sub-directory for target
architecture.
[SCMRequest] Depends on: https://review.tizenrsa.org/74219

[Verification]
1. Build repository.
2. Install widget npruntime-all-on-arm-x86.wgt from
RSA/test/wrt/wrt-package-management-tests/resources/ on the device.
3. Check if only plug-ins for arm architecture have been installed.

Change-Id: Iaa5ddca79f8cb1bc0237531869841864ee0e4625