Invalidate SignalDrawable on level change.

This regression was introduced by ag/7248138. Previously, changes to the
level always caused drawable invalidation, but currently invalidation
only occurs on signal level changes (not cutout or carrier change state
changes). This change fixes this behavior to invalidate the signal
drawable on all level changes.

Fixes: 134621165
Test: sysui demo mode
Change-Id: I5e14480a61fde5a0f7fdce781ae4a42a97e5d88a