Use setEnable() when there is an installed image

We can now use setEnable() to tell GsiService that we would like
run in DynamicSystem on the next restart, if there is a system
image installed.

Bug: 124613126
Test: Tap the button and setEnabe() is invoked.
Change-Id: I74312af0c87548981ecb4e1ac9a1f3814304a4d5