Summary:
1. Provide forceRTL function for developer to test RTL layout in LTR language bundle in I18nUtil and expose it in I18nManager.
2. Rename `allowRTL` and `setAllowRTL` functions
Reviewed By: fkgozali
Differential Revision: D3663693
fbshipit-source-id: 3db13a44c069ae73d1728c211306422db5dd8122