media codec2: Remove use of hardcoded PAGE_SIZE 4096

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 media codec2 source; use instead getpagesize() which
provides the real pagesize.

Test: mma
Bug: 295016003
Change-Id: If70250d4ef1e838756082974abd4d2e41e28f343
Signed-off-by: Kalesh Singh <kaleshsingh@google.com>