Disable bytecode optimization for SystemUIRobo-stub

This target references a number of test-only deps that result in a lot
of spurious conflicting class warnings during bytecode optimization. As
we're not actually using the optimized dex for the Robolectric tests,
disable optimization to avoid such warnings. This should also improve
test build times. Note that we leave shrinking enabled to keep the
generated APK size in check.

Change-Id: I4c5b4d809d5ae686ce12a7a78138e86064a44a5f
Test: atest SystemUiRoboTests
Bug: 222468116
Flag: EXEMPT bugfix