fix(runtime): Unload framework bundles in SymbolLoader::load

Use NSBundle to load bundles instead of CFBundleLoadExecutable
Do not attempt load a bundle more times than it is needed
Unload the bundle if its state was previously unloaded. Sometimes framework bundles use resource bundles of the same name and keeping the framework loaded breaks them. OTH, loading and unloading a framework bundle is sufficient for its executable code to be registered with the Objective-C runtime.

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[x] The PR title follows our guidelines: https://github.com/NativeScript/NativeScript/blob/master/CONTRIBUTING.md#commit-messages.
[x] There is an issue for the bug/feature this PR is for. To avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it.
[x] You have signed the CLA.
[x] All existing tests are passing: https://github.com/NativeScript/ios-runtime/blob/master/CONTRIBUTING.md#contribute-to-the-code-base
[ ] Tests for the changes are included.

refs #1252