Teach clang-format the right import statement order.

Taken from
https://source.android.com/docs/setup/contribute/code-style#order-import-statements.

#codehealth

Test: clang-format on http://ag/20333208
Change-Id: I1378293bbc0a6b7a3482f9bd0beb71f95e11809d

.clang-format

@@ -9,5 +9,17 @@ CommentPragmas: NOLINT:.*
 ConstructorInitializerIndentWidth: 6
 ContinuationIndentWidth: 8
 IndentWidth: 4
+JavaImportGroups:
+- android
+- androidx
+- com.android
+- dalvik
+- libcore
+- com
+- junit
+- net
+- org
+- java
+- javax
 PenaltyBreakBeforeFirstCallParameter: 100000
 SpacesBeforeTrailingComments: 1