WifiNetworkSpecifier: get context reflectively

Temporarily use reflection to get the context
in order to allow framework-wifi to build
against @SystemApi stubs. Will remove reflective
call once refactor is complete to allow
WifiNetworkSpecifier to get the context using
formal APIs.

Bug: 144102365
Test: Ran CTS Verifier NetworkRequest tests, verified package name
in adb logs.
Change-Id: Ib507b449f81e9ce88cdb9190e9f8390eb25819b7