NetworkRequest: Embed requestor uid & packageName

Add the requestorUid & requestorPackageName fields to
NetworkCapabilities. This is populated by CS when
a new network request is received.
These 2 requestor fields are also optionally used for network
matching. All of the regular app initiated requests will have the
requestor uid and package name set by connectivity service. Network
agents can optionally set the requestorUid and requestorPackageName
to restrict the network created only to the app that requested the network.

This will help removing the necessity for the various specifiers to embed
the uid & package name info in the specifier for network matching.

Note: NetworkSpecifier.assertValidFromUid() is deprecated & removed in
favor of setting the uid/package name on the agent to restrict the
network to a certain app (useful for wifi peer to peer API & wifi aware).

Bug: 144102365
Test: Verified that wifi network request related CTS verifier tests
pass.
Test: Device boots up and connects to wifi networks
Change-Id: I207c446108afdac7ee2c25e6bbcbc37c4e3f6529
Merged-In: I207c446108afdac7ee2c25e6bbcbc37c4e3f6529