Fix issue where stylus battery notification shows -1

Bug: 315897903
Fix: 315897903
Flag: NONE Minor fix
Test: Local test on Tangorpro + atest com.android.systemui.stylus.StylusUsiPowerUiTest
Change-Id: I446c24fbe6de5fe87bf5681d91f5d9e402a3f33c
2 files changed