Remove androidx rules from proguard_common.flags

This is following recommendations from [1].
This change has no impact on the APK size.

[1]: go/aperf-keepanno-fixit-faq#heading=h.ymkgf27juo8t
Test: m SystemUI SystemUIGo SystemUIGoogle TvSystemUI CarSystemUI
Test: m CarUiPortraitSystemUI ClockworkSystemUI ArcSystemUI
Test: ensure APK size is identical before and after the change
Bug: 300402748

Change-Id: Ie7edf2c11345f73953070f3a8f660ee711904188