KernelCpuUidBpfMapReader: Verify presence of Uids before deleting

Make sure the first and last Uids are valid exist in the sparse array
before removing the array range of which they are the boundaries.
If they are valid, but not exist, create fake elements to make sure all
the elemens in the range are deleted.

Bug: 157887803
Test: build
Signed-off-by: Alessio Balsini <balsini@google.com>
Change-Id: I3b5ddc499178163cb9b0e9cabbe4f3457ae34dad