Pass --legacy to aapt2 compile for SystemUI

This module produces warnings that are treated as errors if --legacy
is not passed.  --legacy is no longer being passed by default, so
add it here.

Bug: 135597368
Test: m java
Change-Id: I2be35a903b22bfb8e589380cf6e6d582a777b5c8
Merged-In: Ife1b6d77055f5ca3e0b28e8b6b560248b62ad302