ANDROID: block: Prepare for supporting sub-page limits

Introduce variables that represent the lower configuration bounds. This
patch does not change any functionality.

Bug: 308663717
Bug: 319125789
Change-Id: Ia39fbe29a0711caba841a7b44d462608216841ec
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Juan Yescas <jyescas@google.com>