keymaster: Remove usage of PAGE_SIZE

bionic provides PAGE_SIZE macro which happens to also match the
size keymaster send buffer (4096) and half the size of the recv
buffer.

PAGE_SIZE is being removed as no other libc provides this and
Android is moving towards being page-size-agnostic.

Use a 4096 constant instead.

Test: Boot 16k device
Bug: 294914413
Change-Id:  I2dc10b48811e24d25ba08cfe4ffb514e94d42a8f
Signed-off-by: Kalesh Singh <kaleshsingh@google.com>