sign_target_files_apks: Fix password encrypted keys handle

This patch restores the possibility of using password encrypted
keys for build signing.

Bug: 171221825
Test: 1. Generate password encrypted keys (write non empty passwords):
        $ subject='/C=US/ST=California/L=Mountain View/O=Android/OU=Android/CN=Android/emailAddress=android@android.com'
        $ mkdir ~/.android-certs
        $ for x in releasekey platform shared media; do \
            ./development/tools/make_key ~/.android-certs/$x \
            "$subject"; \
          done
      2. Create a file with passwords:
        Example of passwd file:
            [[[ 12345678 ]]] /home/user/.android-certs/releasekey
            [[[ 12345678 ]]] /home/user/.android-certs/platform
            [[[ 12345678 ]]] /home/user/.android-certs/shared
            [[[ 12345678 ]]] /home/user/.android-certs/media
            [[[ 12345678 ]]] /home/user/.android-certs/networkstack
      3. Tell system where to find passwords:
        $ export ANDROID_PW_FILE=/path/to/file/with/passwords
      4. Generate a release image:
        $ make dist
        $ sign_target_files_apks \
        -o --default_key_mappings ~/.android-certs \
        out/dist/*-target_files-*.zip \
        signed-target_files.zip

Signed-off-by: Oleh Cherpak <oleh.cherpak@globallogic.com>
Change-Id: I3e9d5318f69a2c3ac6eec64b36163b6544d49c90