Update repo init instructions to use --partial-clone
Update repo init instructions to use --partial-clone

This allows repo checkout to be smaller than the default allowing for quicker checkout.

Change-Id: I5268c3ccaee1bd86a2ea21c06413aa6df48129ea
Test: local repo init and sync

AndroidX/androidx,androidx/androidx,androidx/androidx,AndroidX/androidx,AndroidX/androidx,androidx/androidx,AndroidX/androidx,AndroidX/androidx,androidx/androidx,androidx/androidx,AndroidX/androidx,AndroidX/androidx,androidx/androidx,androidx/androidx,AndroidX/androidx,androidx/androidx,AndroidX/androidx,androidx/androidx,AndroidX/androidx,androidx/androidx