Revert^2: Greylist com.google.vr.platform hidden methods

Enforcement of non-SDK API is being extended to all modules on the
boot class path. This includes the com.google.vr.platform package,
which is used to set up VR on a given device. Two hidden methods
are used by the client static library via reflection. Mark them
@UnsupportedAppUsage so that they remain discoverable.

Bug: 111911841
Test: make
Change-Id: Iaed1cd6bc7419597510bfc1ab30a169f75e4ae3f