Reason for the refactor is an upcoming change to download/parse the
payment manifest on desktop in addition to Android.

1) This patch moves downloader and parser utility into cross-platform
   directory //components/payments/content.

2) The cross-platform PaymentManifestParserHost has been separated from
   its Android wrapper PaymentManifestParserAndroid.

3) GN build target renames:
   a) Payments mojom targets have been renamed to include the string
      "mojom" for clarity.
   b) The main target of "//components/payments/content" has been
      renamed to "content" for brevity.
   c) The "//components/payments/content:payment_validators" target has
      been renamed to "//components/payments/content:utils", because it
      now contains manifest downloader and parser.

Manifest download is currently used only on Android after enabling the
chrome://flags/#android-payment-apps flag.

BUG=683329

Review-Url: https://codereview.chromium.org/2759283002
Cr-Commit-Position: refs/heads/master@{#460141}