TI: plugin_store: 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 [1]

Remove the usage of this hard-coded value by using getpagesize() instead
as recommended by the doc [2].

[1] https://source.android.com/docs/core/architecture/16kb-page-size/16kb
[2] https://source.android.com/docs/core/architecture/16kb-page-size/getting-page-size


Signed-off-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>