Refactor device_manager

This refactors the device_manager property to avoid calling is_android_build unnecessarily each time device_manager is referenced.

is_android_build calls marionette.session_capabilities.  When, for example, cleanUp tries to delete 1200 media files (during perf tests, for example), this is 1199 unnecessary calls to marionette.session_capabilities, which significantly slows things down.