Ignore empty maps when emitting DWARF variable locations.

This is rewrite of https://android-review.googlesource.com/#/c/202115

The aim in both cases is to avoid gaps in generated locations.
(which helps to keep the size of the generated DWARF down).

However, the previous CL was a bit too eager with extending of
variable scope and reporting locations. We might have reported
variable in scope when in fact, it was not.

This CL implements simpler solution by just filtering stackmaps
without dex register maps at first opportunity. This should
ensure that locations for breakpoint locations are completely
accurate as originally intended.

Change-Id: I98378716c0ef5ef46b12181502904621eb6ecf2f