fs_mgr: overlayfs: CreateDynamicScratch for *partition_exists

Commit 69def12f
("overlayfs: Refactor how the scratch device is deduced, again.")
introduced a regression which dropped a call to
dm.GetDmDevicePathByName.  The call was formerly done to check if
a partition existed and shortcuted the logic.  As a result when
the partition exists and is already instantiated, CreateDynamicScratch
function fails to fill out *scratch_device return value.

The bandaid is to call GetBootScratchDevice() to leverage its own
GetDmDevicePathByName call to fill out if it is detected as an empty
value in CreateDynamicScratch, only called under this condition.

Signed-off-by: Mark Salyzyn <salyzyn@google.com>
Test: adb-remount-test.sh
Bug: 148881519
Change-Id: I40464e8dd17d25a292a275d0b9df5f9519b794fb