Never instatiate App class multiple times

- GMS-core is still sometimes seeing the app class being
instatiated multiple times.

Let's just cache the app instance to avoid such situations.

- Also removed some tentative WTFs, which have never been triggered.
(so they should be unrelated to the dup-app bug)

Bug: 185177290
Test: Boot / treehugger
Change-Id: I4c74a5798264472d1bfbf5604f399a49e5946c64