Use FrameAllocator rather than Heap for shared memory pool.

Heap uses an intrusive linked list to store metadata inside unallocated
memory. This could be modified by a malicious host to manipulate the
allocator, which we don't want.

Bug: 280644106
Test: m pvmfw
Change-Id: I75903084da8f4f2c12063b685d41aa1c8ee37da3
Merged-In: I75903084da8f4f2c12063b685d41aa1c8ee37da3