[frameworks][native][vulkan] fix -Walloca

Alloca cannot be checked for failure. Replace alloca with dynamic memory
allocations.

Reapply a commit that was previously reverted.  The original commit
removed a call to enumerate_instance_layers() that it should not have.

Prefer std::vector to std::unique_ptr of an array.

Bug: 139945549
Bug: 142475221
Bug: 143156243
Test: mm && adb shell stop && adb sync && adb shell start && atest -it \
  CtsGpuToolsHostTestCases:android.gputools.cts.CtsRootlessGpuDebugHostTest#testDebugLayerLoadExternalVulkan,testMultipleExternalApps,testSystemPropertyIgnoreVulkan,testDebugLayerLoadVulkan,testSystemPropertyEnableVulkan
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>

Change-Id: Ifdf747fdabc41ee6da8cd83bda5e3f030649030f