init: fix microdroid specific mishandling of powerctl arguments

The second argument doesn't have to be a reboot target. `reboot,$REASON`
is a common pattern in android. For code simplicity, we just skip reboot
target processing entirely for now. Eventually we should find a nicer
way to turn off functionality for microdroid.

Bug: 433870205
Test: atest MicrodroidTestApp 'com.android.microdroid.test.MicrodroidTests' --test-filter '.*bootFailsWhenBinaryNameIsInvalid.*'
Flag: EXEMPT .
Change-Id: I81b2439ef08bc2d471ab288ee9ce9123f5961b0d