Fix NPE race condition in EmbeddedSessionManager.updateDisplayCountAndDuration()

[Shamyyoun]

Summary

Fixes a NullPointerException crash in updateDisplayCountAndDuration() caused by a TOCTOU (time-of-check/time-of-use) race condition.

Closes #1052

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Problem

EmbeddedImpressionData.start is a plain var Date? with no synchronisation. The previous code checked if (impressionData.start != null) but then accessed impressionData.start!! in the body. When called concurrently from multiple threads (e.g. via Dispatchers.Default coroutines), another thread can set start = null between the null-check and the dereference, causing a crash:

Fatal Exception: java.lang.NullPointerException
  at com.iterable.iterableapi.EmbeddedSessionManager.updateDisplayCountAndDuration(EmbeddedSessionManager.kt:114)
  at com.iterable.iterableapi.EmbeddedSessionManager.endAllImpressions(EmbeddedSessionManager.kt:91)
  at com.iterable.iterableapi.EmbeddedSessionManager.endSession(EmbeddedSessionManager.kt:41)
  at kotlinx.coroutines.scheduling.CoroutineScheduler$Worker.run(CoroutineScheduler.kt:704)

Fix

Capture impressionData.start into a local val before the null-check. Because local variables are thread-confined (stack-allocated), the value cannot be changed by another thread after the null-check passes:

// Before
if (impressionData.start != null) {
    impressionData.duration = ... - impressionData.start!!.time  // ← NPE under concurrent access
}

// After
val start = impressionData.start
if (start != null) {
    impressionData.duration = ... - start.time  // ← safe, local val is immutable
}

This is the idiomatic Kotlin/Java pattern for safe null-check-then-use on shared mutable fields.