check-flagged-apis: parse API versions XML

Teach check-flagged-apis to parse API versions XML; this represents the
APIs after metalava has processed the source and kept APIs as is, or
reverted them to the previous SDK snapshot, according to their
@FlaggedApi flags.

As with the API signature parser, limit support to fields to keep things
simple; support for classes and methods will be added in later CLs.

Note: `m sdk dist` will generate an API versions XML file.

Bug: 334870672
Test: atest --host check-flagged-apis-test
Test: check-flagged-apis --api-signature out/target/product/mainline_x86/obj/ETC/frameworks-base-api-current.txt_intermediates/frameworks-base-api-current.txt --flag-values out/soong/.intermediates/all_aconfig_declarations.pb --api-versions out/dist/data/api-versions.xml
Change-Id: I779a0d0cdb8a50536d3fc8d517fa38ba4b0dcd1c

tools/check-flagged-apis/src/com/android/checkflaggedapis/CheckFlaggedApisTest.kt

@@ -52,6 +52,18 @@ private val PARSED_FLAGS =
       ByteArrayInputStream(binaryProto.toByteArray())
     }()
 
+private val API_VERSIONS =
+    """
+      <?xml version="1.0" encoding="utf-8"?>
+      <api version="3">
+        <class name="android/Clazz" since="1">
+          <method name="&lt;init>()V"/>
+          <field name="FOO"/>
+        </class>
+      </api>
+"""
+        .trim()
+
 @RunWith(DeviceJUnit4ClassRunner::class)
 class CheckFlaggedApisTest : BaseHostJUnit4Test() {
   @Test
@@ -67,4 +79,11 @@ class CheckFlaggedApisTest : BaseHostJUnit4Test() {
     val actual = parseFlagValues(PARSED_FLAGS)
     assertEquals(expected, actual)
   }
+
+  @Test
+  fun testParseApiVersions() {
+    val expected: Set<Symbol> = setOf(Symbol("android.Clazz.FOO"))
+    val actual = parseApiVersions(API_VERSIONS.byteInputStream())
+    assertEquals(expected, actual)
+  }
 }