Make undefined ThemeBundle propagatable and buildable

- We won't need to pass the boolean value of ThemeBundle#isDefined.
- This also fix the problem of wrong custom theme suggestion: https://screenshot.googleplex.com/zEEX5WaaNgZ.png
Because the Custom button and Default theme are with empty packges set, and Custom button is in front of Default in the list. So add #isDefined to #isEquivalent to ensure Custom button has no equivalent.
Fix screenshot: https://screenshot.googleplex.com/SvUNccCWjYD.png

Test: Manually
Fixes: 157723282
Change-Id: I6a5f75840edd2153f2b197468762dda80fb13e8c