Bump the minSdkVersion from 29 to 30

DocumentsUI doesn't get released to Android 10 (Q) with Android 11
(R) being the oldest version it supports. Let's bump the supported
SDK version to support this.

Bug: 395794975
Test: Ensure DocumentsUI builds
Flag: EXEMPT resource change
Change-Id: Ic7035dfe7a9cccbd93ea4b022c9b4f17e52cbd5c