protoutil: EncodedBuffer: Remove usage of 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 protoutil source; use instead getpagesize() which
provides the real pagesize.

Test: atest -c libprotoutil_test
Bug: 295228590
Change-Id: I6c3212600256d61a71430dfbbf7b0c9944fe4859
Signed-off-by: Kalesh Singh <kaleshsingh@google.com>