Add app-level usesCleartextTraffic manifest attribute.

The attribute declares whether the app intends to use cleartext
network traffic (e.g., HTTP, WebSockets, XMPP, SMTP, IMAP -- without
TLS or STARTTLS). The default value is true. If set to false, the app
declares that it does not intend to use cleartext network traffic. In
this case the app requests the platform, tooling, and third-party
libraries to prevent it from using cleartext traffic. The danger of
cleartext network traffic is that its confidentiality, authenticity,
and integrity are not guaranteed.

This feature is designed to help apps which care about security of
data exchanged over the network. These apps can accidentally
regress/downgrade to using cleartext network communications. This
typically happens when the server the app communicates with all of a
sudden tells it to use cleartext communications (e.g, HTTP URL
instead of an HTTPS URL) or when one of the components of the app gets
updated and regresses to cleartext communications without the
developer noticing.

In general, the prevention measures are on best effort basis. It's
impossible to automatically prevent all instances of cleartext
traffic. For example, an app bent on bypassing restrictions could
perform low-level network I/O with unusual TCP packet fragmentation,
or could use a custom application-level protocol.

The expectation is that most apps use libraries for network
communications and these libraries over time will start to honor this
flag, thus increasing the protections offered by it.

Bug: 19215516
Change-Id: I8700d51ddbc5d528faa4b6a5fa5bc9551ad02d13