Use apexd_host for host-side APEX extraction

Host-side simulation of APEX activation is done by a new tool
'apexd_host'. This simplies checkvintf invocation for local builds and
for target-files.

For local builds, checkvintf no londer depends on $OUT/apex, a flattened
view of APEXes. In fact, the build system doesn't need to install
$OUT/apex. They are installed for now only for its side-effect of
installing symbol files. We'd better not rely on $OUT/apex.

For target-files, scanning/activating apexes are extracted and moved to
the new tool. Now check_target_files_vintf is not more efficient because
it doesn't copy .apex files to a temporary directory.

Bug: 260358957
Bug: 288826922
Test: m (running checkvintf) # for local builds
Test: m target-files-package && check_garget_files_vintf target-files.zip
Test: atest releasetools_test
Change-Id: Iba23f429d96f9ec31814196aa14bdbb800649218