Detect duplicate ANDROID_EXTRA_LIBS and minor bug fix

Summary:
Don't include same dependency twice, one which was found
from readelf in specifydependencies.cmake and other in
ANDROID_EXTRA_LIBS.

Bug fix: find stl was dependent on the order in which libc++
was added, now it is independent.

Test Plan: * add the same dependency in ANDROID_EXTRA_LIBS

Reviewers: apol, vkrause

Reviewed By: apol

Subscribers: kde-buildsystem, kde-frameworks-devel

Tags: Frameworks, Build System

Differential Revision: https://phabricator.kde.org/D20509