Make time remaining estimate debug mode more reliable.

Previously, the long click handler was unregistered on first use and
never re-registered. This meant that, e.g., on plugging and unplugging
the phone, debug mode stopped working. This change moves long click
activation into restartBatteryInfoLoader().

Bug: 63133793
Test: make RunSettingsRoboTests
Change-Id: I4c25a1f05ab3718bf06faf455cf670b5460a7306