Enable referencing of aapt-generated proguard flags

Some targets may implement classes declared elsewhere, e.g., system
server implements services defined in framework-res.apk's manifest.
Allow depending on the aapt-generated proguard flags for a given
target to support this.

Bug: 272495195
Test: m + reference ":framework-res{.aapt.proguardOptionsFile}"
Change-Id: I2a16632ed6e5a9bddbe326bdb7ab0dd79b45e587