Introduce script and module to pass aconfig flags to metalava

`keep-flagged-apis` module generates a txt file that
contains annotations based on aconfig flags that are passed to metalava
when generating "exportable" or "runtime" stubs.

`keep-flagged-apis.sh` already exists under
`packages/modules/SdkExtensions` directory, but modifying the script
(and the bp module definition) will break the `udc-mainline-prod` branch
in downstream. Therefore, the modified script and the module definition
is defined here, and will be used in the long term.

Test: m keep-flagged-apis && manual
Bug: 315485740
Change-Id: I4ab8d4ea2c6c8ddba6d55b2b25e8c8d3b7c8fa07

scripts/Android.bp

@@ -254,3 +254,8 @@ python_test_host {
         "modify_permissions_allowlist.py",
     ],
 }
+
+sh_binary_host {
+    name: "keep-flagged-apis",
+    src: "keep-flagged-apis.sh",
+}

scripts/keep-flagged-apis.sh

@@ -0,0 +1,46 @@
+#!/bin/bash -e
+#
+# Copyright 2023 Google Inc. All rights reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+# Convert a list of flags in the input file to a list of metalava options
+# that will keep the APIs for those flags will hiding all other flagged
+# APIs.
+
+FLAGS="$1"
+
+FLAGGED="android.annotation.FlaggedApi"
+
+# Convert the list of feature flags in the input file to Metalava options
+# of the form `--revert-annotation !android.annotation.FlaggedApi("<flag>")`
+# to prevent the annotated APIs from being hidden, i.e. include the annotated
+# APIs in the SDK snapshots. This also preserves the line comments, they will
+# be ignored by Metalava but might be useful when debugging.
+while read -r line; do
+  key=$(echo "$line" | cut -d= -f1)
+  value=$(echo "$line" | cut -d= -f2)
+
+  # Skip if value is not true and line does not start with '#'
+  if [[ ( $value != "true" ) && ( $line =~ ^[^#] )]]; then
+    continue
+  fi
+
+  # Escape and quote the key for sed
+  escaped_key=$(echo "$key" | sed "s/'/\\\'/g; s/ /\\ /g")
+
+  echo $line | sed "s|^[^#].*$|--revert-annotation '!$FLAGGED(\"$escaped_key\")'|"
+done < "$FLAGS"
+
+# Revert all flagged APIs, unless listed above.
+echo "--revert-annotation $FLAGGED"