Add config param to disable app snapshots.

There is an upcoming 'slim' emulator image which aims to reduce
resource consumption when running headless tests, largely by
disabling GL draw calls and allocations. App snapshots result in GL
draw calls, so its desirable to have ability to turn these off by
default in the slim image.

Bug: 188559292
Test: launch emulator, manually verify snapshots
Change-Id: I83d9ab4a15a862ea220debdd8da8b3d5673da276