Make NotificationMemoryViewWalkerTest pass on cf tablet

Reduce the sizes of bitmaps used by this test to make them pass with the
latest cuttlefish screen configurations. Currently they are failing,
because these devices have a density, which makes to resize these
bitmaps as their size is bigger than a max dimens value in pixels.

Bug: 306164991

Test: abtd runs on different targets
Change-Id: I2cc55c27cedb7066089e110257dac0170a32e9f1