mediautils: SharedMemoryAllocator: Remove PAGE_SIZE 4096 usage

bionic hard codes the PAGE_SIZE macro as 4096. This is going away as
Android begins to support larger page sizes. Remove the usage of this
assumption from shared memory allocator; use instead getpagesize()
which provides the real pagesize.

Test: atest -c shared_memory_allocator_tests
Bug: 295228437
Change-Id: I69b1f88de096b0b9fec4ed829c2fb4ce6dddd347
Signed-off-by: Kalesh Singh <kaleshsingh@google.com>