Rename test_only_no_hashtree to generate_hashtree

In order not to introduce any changes in the system image default value
of generate_hashtree is currently set to false. Once all the APEXes that
shouldn't have hashtree enable (i.e. non-updatable ones) explicitly set
`generate_hashtree: false` default value will be switched to true.

Test: checked that are no diffs between ${OUT}/system/apex without this
  change and with this change.
Test: m
Bug: 190621617
Change-Id: I5a3144f8f6ea7dee25678af43e2b07a8c85daa9e