Keep PRODUCT_FULL_TREBLE to true even when some requirements are not met

When some of the full treble requirements are turned off,
PROUCT_FULL_TREBLE became false, which in turn cleared macro variables
guarded in treble: {cflags: "..."}.

Until when Soong recognizes each of the requirements (e.g.
treble_sepolicy: { }), don't reset PRODUCT_FULL_TREBLE to false
even when some of the requirements are not met.

Bug: 62019611
Bug: 68710251
Test: get_build_var PRODUCT_FULL_TREBLE returns true for aosp_walleye
Test: aosp_walleye boots to the UI
Change-Id: Iad3ee4f4462c82e9f9e1897f3ec019cdd16da358