Switch to aapt2 for getting minSdkVersion

aapt does not support parsing some referencing attributes.
Switch to latest tool, aapt2, which works better.

Test: aapt2 dump badging test.apk and returned 0.
Test: Run check_target_files_signatures.py with a target_files.zip.
Test: Run sign_target_files_apks.py with a target_files.zip.
Change-Id: Ib3a4740506f29ebab6930767f2aa8a0b5c4ba053