Support release configs with only aconfig flags

Release configs with `aconfig_flags_only: true` can inherit build flag
values, but cannot set them.

Bug: 328495189
Test: manual
Ignore-AOSP-First: cherry-pick
Merged-In: I2bdc88761d61fb20f10dc734f2ba87114f51b859
Change-Id: I2bdc88761d61fb20f10dc734f2ba87114f51b859