Fix ever-growing AppOpItem string representation

Test: Manual: `adb shell dumpsys activity service com.android.systemui \
| grep AppOpItem` lines end with `Paused=false)` or `Paused=true)` even
when run multiple times, not gathering additional `false)` or `true)`.

Change-Id: I073e6c7c8180b9bd76d45570596a9b2ba769bbe3