Add new testcase in IpConfigStoreTest to cover the readIpConfiguration
usage, improve the code coverage for connectivity module.
To reduce the flakyness of test, this cl refactors DelayedDiskWriter
constructors by allowing to pass in a Dependencies instance, then
waitForIdle can be used to check if the delayed write operation
associated with the passed-in HandlerThread has completed in tests.
Bug: 234315786
Test: atest com.android.server.net.IpConfigStoreTest
Change-Id: I57c8bd02a771f1cca815252ba502cda0210d1385
b4a8fd534a