Rename sdk field tag "keep" to "ignore"

"ignore" is a better description of the effect that this tag has on the
optimization that is done as part of the sdk snapshot generation.
"keep" implies that something special is done with the field during the
optimization process but actually the optimization process completely
ignores the property, just as it does with unexported fields.

Bug: 248258460
Test: m nothing
Change-Id: Idfdfd87f927d641d4d301cbbde72b463c89155bf
diff --git a/android/sdk.go b/android/sdk.go
index a9cc547..42f95b7 100644
--- a/android/sdk.go
+++ b/android/sdk.go
@@ -735,8 +735,8 @@
 	//   have common values. Those fields are cleared and the common value added to the common
 	//   properties.
 	//
-	//   A field annotated with a tag of `sdk:"keep"` will be treated as if it
-	//   was not capitalized, i.e. not optimized for common values.
+	//   A field annotated with a tag of `sdk:"ignore"` will be treated as if it
+	//   was not capitalized, i.e. ignored and not optimized for common values.
 	//
 	//   A field annotated with a tag of `android:"arch_variant"` will be allowed to have
 	//   values that differ by arch, fields not tagged as such must have common values across
@@ -923,18 +923,18 @@
 	// the locations of any of their prebuilt files in the snapshot by os type to prevent them
 	// from colliding. See OsPrefix().
 	//
-	// This property is the same for all variants of a member and so would be optimized away
-	// if it was not explicitly kept.
-	Os_count int `sdk:"keep"`
+	// Ignore this property during optimization. This is needed because this property is the same for
+	// all variants of a member and so would be optimized away if it was not ignored.
+	Os_count int `sdk:"ignore"`
 
 	// The os type for which these properties refer.
 	//
 	// Provided to allow a member to differentiate between os types in the locations of their
 	// prebuilt files when it supports more than one os type.
 	//
-	// This property is the same for all os type specific variants of a member and so would be
-	// optimized away if it was not explicitly kept.
-	Os OsType `sdk:"keep"`
+	// Ignore this property during optimization. This is needed because this property is the same for
+	// all variants of a member and so would be optimized away if it was not ignored.
+	Os OsType `sdk:"ignore"`
 
 	// The setting to use for the compile_multilib property.
 	Compile_multilib string `android:"arch_variant"`