releasetools: Skip signing APKs with given prefixes.

We may pack prebuilts that end with ".apk" into target_files zip, via
PRODUCT_COPY_FILES. META/apkcerts.txt won't contain the cert info for
such files, and we want to keep them as is while signing, despite of the
".apk" extension.

This CL adds "--skip_apks_with_path_prefix" option to
sign_target_files_apks.py. APKs with matching prefixes will be copied
verbatim into the signed images. The prefix should match the entry names
in the target_files (e.g. "SYSTEM_OTHER/preloads/"). The option may be
repeated to specify multiple prefixes.

Note that although we may skip signing an APK file with "-e ApkName=".
This would skip *all* the APK files with the matching basename.
"--skip_apks_with_path_prefix" allows matching the exact prefix.

For example:
$ ./build/make/tools/releasetools/sign_target_files_apks.py     \
    --skip_apks_with_path_prefix SYSTEM_OTHER/preloads/         \
    --skip_apks_with_path_prefix PRODUCT/prebuilts/PrebuiltApp1 \
    --skip_apks_with_path_prefix VENDOR/app/PrebuiltApp2.apk    \
    target_files.zip                                            \
    signed-target_files.zip

Bug: 110201128
Test: Run the command above and check the logs.
Test: `python -m unittest test_sign_target_files_apks`
Change-Id: I7bd80b360917cef137cf1e7e8cfa796968831f47