Don't use platform cert in CaptivePortalLoginTests

The test can run fine with shell permissions: this allows running the
test on more devices; for example compiling the test from AOSP and
running it on a signed build, or testing various OEM devices.

Also set min_sdk_version to 29, which allows compiling the test from a
development branch and running it on a device with a released SDK.

Bug: 162296431
Test: atest CaptivePortalLoginTests
Change-Id: Ic17c09a4c62770ceef7a6d8265232e173de3c9da
3 files changed