System popup covers Adder on mobile

**Describe the bug**
Sometimes the Andriod system popup that appears when selecting text covers the Hypothesis adder.

**To Reproduce**
Uncertain. Try:
1. Use Via on Android to access a page
2. Select text on page and observe Adder and system popup

**Expected behavior**
The Hypothesis Adder should appear over or away from the system popup.

**Screenshots**

**Smartphone (please complete the following information):**
- Device: Asus Zenfone 6
- OS: Android 10

**Additional context**
https://hypothesis.zendesk.com/agent/tickets/10230

The issue provides no source file, test, or entry point to inspect. Begin by reproducing the behavior in Via on Android with text selection, then locate the Hypothesis Adder's mobile positioning behavior. Done means the Adder no longer overlaps the Android system text-selection popup in the reported scenario.

Written by the indexing model from the issue text.