Update the doc for the MediaProjectionManager#getMediaProjection

So it reflects the latest requirement from Android U regarding
the mediaProjection FGS permission, which was introduced in
the I1e5b58ddea232fecaaa5ed513276a62c72429a0b.

Bug: 269758761
Test: m offline-sdk-docs
Change-Id: I9b52f2227e7745886a238a3ef96f704d32788dd9