Stop using invalid URL to prevent unexpected crash

Verify the input PAC Uri before performing follow-up actions.

Check if the URL is a valid URL to filter some invalid URLs since
these invalid URLs could not fall into any subclass of existing
URLConnections. When the PAC Uri is other invalid URL scheme, it
will cause an UnsupportedOperationException if there is no proper
subclass that implements the openConnection() method.
A malformed URL may crash the system.

Even it's a valid URL, some subclasses(e.g. JarURLConnection)
may not have openConnection() implemented. It will also hit the
problem, so convert the possbile exception from openConnection()
to re-throw it to IOException which is handled in the existing
code.

Bug: 219498290
Test: atest FrameworksNetTests CtsNetTestCases
Test: Test with malformed URL
Merged-In: I22903414380b62051f514e43b93af992f45740b4
Merged-In: I2abff75ec59a17628ef006aad348c53fadbed076
Change-Id: I4d6cec1da9cf3f70dec0dcf4223254d3da4f30a3
(cherry picked from commit 6390b37a)