Fix incorrect index in DefaultAllocator.trim

This could cause us to "lose" allocations backed by an
initial block, meaning they became unavailable for use
despite still being allocated.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=131931465

library/src/main/java/com/google/android/exoplayer2/upstream/DefaultAllocator.java

@@ -134,7 +134,7 @@ public final class DefaultAllocator implements Allocator {
         if (lowAllocation.data == initialAllocationBlock) {
           lowIndex++;
         } else {
-          Allocation highAllocation = availableAllocations[lowIndex];
+          Allocation highAllocation = availableAllocations[highIndex];
           if (highAllocation.data != initialAllocationBlock) {
             highIndex--;
           } else {