Enforce MEDIA_CONTENT_CONTROL in verifyPackageName

Also rename verifyPackageName to verifyPackageExists,
which probably reflects behavior a bit better.

Before this change, an app can use a direct binder
call to ask the system whether a package is installed
without having any permissions. Since the method now
requires MEDIA_CONTENT_CONTROL, we don't need
enforceMediaContentControlPermission anymore.

Bug: 205124386
Test: atest MediaRouter2HostSideTest CtsMediaBetterTogetherTestCases
Change-Id: I66723a00cf8b22b40aa0cd680a7dbe4bb71fddb6