APK signature verification: use FileChannel#size

RandomAccessFile#length returns 0 for block devices while
FileChannel#size returns the correct size.

APK signature verification utils should work with block-device zip files
when Android is running as a guest OS and APK/APEX files are passed via
block devices.

This is a follow-up of 1257f150.

Bug: 193592496
Bug: 192366904
Test: FrameworksServicesTests
Change-Id: I3015fc30574c36c18c0d0c9ae7453d33a1349927
(cherry picked from commit 310edbe3)