Prevent the BrightLineFalsingManager from duping its session.

The BrightLineFalsingManager should not be able to start a session
when it's already in a session. Primarily, this caused the
FalsingManager to hang onto extra registrations to the Proximity
Sensor, per the bug.

Bug: 138220274
Test: atest SystemUITests and manual.
Change-Id: Id10d2697a96524e98c87aaa87702209d1752fe68
(cherry picked from commit 0ae8f2e3)