Reorder test expectations and use AtomicBoolean in postMessageTest

This test had ordering issues between when a state was set and assigning
a non-null value to mSession. Also swapped with an AtomicBoolean since
the boolean is set on UI thread.

BUG=35325754
Test: android.support.customtabs.PostMessageTest

Change-Id: Iaa10d28b6f40060eddffc673fcb749036599d118
1 file changed