Add android:angle on <gradient> for min sdk <= Q

The default orientation of gradients in android Q is different than
previous android versions. Set the android:angle attribute to "0"
to ensure that the default gradient orientation will remain
left-to-right in android Q.

Bug: 139883313
Test: aapt2_tests
Change-Id: I82e222f635858ed3ecf4a3f3761503d857adb93b