remount: Do not run the create-scratch logic on older devices.

The prologue of fs_mgr_overlayfs_create_scratch() will implicitly
succeed on physical block devices, and implicitly fail if for some
reason they can't be accessed.

This patch makes the success and failure cases explicit. The logic
specific to DAP has been moved to CreateDynamicScratch.

fs_mgr_overlayfs_create_scratch now calls GetScratchStrategy, and only
calls CreateDynamicScratch for DAP-launch devices. In the case a
physical block device can be used, no action is taken.

Bug: 134949511
Test: adb remount and adb_remount_test.sh
Change-Id: I0af7cda9bc551416c9e2ffca5a36305f89d3bf46