vkgetSwapchainGrallocUsageANDROID() now sets
a single flag to GRALLOC_USAGE_SW_WRITE_OFTEN

Bug: b/122837237
Change-Id: I809659c6bbc49b84302b8c58751de5ed0135b1ef
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/32249Tested-by: Hernan Liatis <hliatis@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Chris Forbes <chrisforbes@google.com>