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: I2ac9d91ad24194c3205adad1d333649bfd3d2bec