stagefright/foundation: Increase AMessage max item count to 256

At the same time replace fixed array implementation with std::vector
to avoid overallocation as typical AMessage size is 0-2 items.

Still keeping the CHECK behavior of trying to increase the AMessage
size beyond the max item count. This will be revisited in the future.

Bug: 190795569
Test: atest sf_foundation_test
Test: video recording and YT playback
Change-Id: I1bd0378db4587fefdabac11c0ede7344f9c1714a