Add volatile to some cancel compaction signals to avoid compiler optimizations

The compaction cancel signals are delivered between different threads so
add volatile keyword for such variables to make sure the compiler does
not try to optimize the values avoiding them to be effective due to
potential register caching.

Bug: 227060330
Test: Manual

Change-Id: I42503e02627c6f76382a58da9320b3f5df15a75d