surfaceflinger: fix race conditions in captureScreen

The display was looked up without holding the state lock.  It was
also used (including indirectly from updateDimensions) without
holding the lock either.

Inline updateDimensions in captureScreen with the state lock held.
Stop calling updateDimensions in captureLayers.

Bug: 113041375
Test: take screenshot, rotate screen, screencap
Change-Id: I8b361847c44373ce08930d906ec4a995efd1c21b