Change the hashtree hash algorithm to sha256

Bug: 155771970
Test: build an arbitrary APEX (e.g. com.android.runtime)
$ unzip com.android.runtime.apex
$ avbtool info_image --image apex_payload.img | grep Hash
    Hashtree descriptor:
      Hash Block Size:       4096 bytes
      Hash Algorithm:        sha256
Test: compare the APEX size before and after the change: no difference
Change-Id: I8924349287d38a72be1058424a100c1981f656d1
1 file changed