KernelCpuUidBpfMapReaderTest: test UID removal with absent UIDs

The original implementation of
KernelCpuUidBpfMapReader.removeUidsInRange did not work unless data
was present for the first & last UIDs in the range. Add a test case to
verify the fix for this issue & prevent regressions

Test: confirm test passes with fix & fails without it
Bug: 157887803
Change-Id: Icfabb7298dbfb1a7f1b89dadbe33bae0492922b6
Signed-off-by: Connor O'Brien <connoro@google.com>