[SP20] Check signature permission when accessing network stats provider

Currently, registerNetworkStatsProvider requires the
UPDATE_DEVICE_STATS permission. This is a privileged permission
so it can be granted to preinstalled apps. Thus, apps like
GmsCore, or preinstalled apps will be able to update network stats.

This change checks for a new permission that would only allow
signature apps to declare that. Also check
MAINLINE_NETWORK_STACK permission to allow NetworkStack process
to use it.

Test: adb shell dumpsys netstats
Test: atest FrameworksNetTests
Bug: 149652079
Change-Id: Idfebd0a1988c3dcfd812d87e30f6a2034d6fbf6b