Rename ro.emulated_storage properties.

Whether these are enabled or not is independent of whether storage is
emulated or not; instead, use 'external_storage', and add the enabled
suffix as per property naming guidelines.

Bug: 152170470
Bug: 153525566
Test: N/A
Change-Id: Id893ce19025ded1ee1521081f7fc722dfee8e662
Merged-In: Id893ce19025ded1ee1521081f7fc722dfee8e662