Support deterministic output for bootimg/filesystem

Adding salt to bootimg/filesystem so that avbtool can produce the same
output with same input.

Adding timestamp/uuid to filesystem so that resulting image can be
deterministic.

Bug: 229784558
Test: m com.android.virt
      # remove intermediates and touch some sources
      m com.android.virt
      # compare two built artifacts
Change-Id: I4e4668fd0ac42a35bea5a33ec3ae8c362b6a6bd2