Make Condition#start a suspend function.

This change makes Condition#start a suspend function so that
suspendable calls can be made within the function body. It also
converts existing conditions to Kotlin to more easily extend start.

Test: atest ConditionTest
Bug: 394520234
Flag: EXEMPT refactor
Change-Id: I877ec08f454d18b5b55c4005039b46ba52011429