Soong: remove output file before running signapk

It looks like there is a bug in rewrapper/reproxy that makes it upload the
output file as an input if it still exists in the output tree. This causes
unnecessary cache misses on incremental builds, and can also cause the
action to fail remotely (depending on the service). By removing the output
file at the beginning of the action, we ensure that it can't be uploaded.

Test: ran against a remote execution server; actions no longer fail remotely
Change-Id: Iaac2b23b35d2c876c0244edbfd4a8f159df0563c